Alt Text SEO: The Overlooked Signal Most Sites Get Wrong

Alt text SEO is the practice of writing descriptive alternative text for images so that search engines can interpret visual content and index it appropriately. Done well, it contributes to image search visibility, reinforces topical relevance on a page, and improves accessibility for users relying on screen readers. Done badly, it is either stuffed with keywords or left completely blank, and both approaches cost you.

Most sites treat alt text as an afterthought, which is exactly why it remains one of the more accessible SEO improvements available. The competition for image search real estate is lower than it should be, and the technical barrier to fixing alt text is close to zero.

Alt text sits within a broader set of on-page decisions that determine how well a page performs in search. If you want to see how these individual signals connect into a coherent approach, the Complete SEO Strategy hub covers the full picture, from technical foundations through to content and link acquisition.

Why Do Search Engines Need Alt Text at All?

Search engine crawlers process text far more reliably than they process images. Google has made significant progress with visual understanding through machine learning, and it can make reasonable inferences about what an image contains. But reasonable inferences are not the same as confirmed context, and confirmed context is what drives confident indexing.

When you write alt text, you are removing ambiguity. You are telling the crawler what the image shows, which reinforces the topical signals already present in your headings, body copy, and structured data. Google weighs all of these signals together when deciding what a page is about and where it should rank.

The image search index operates slightly differently from the main web index. For an image to appear in Google Images, it needs enough contextual information to be matched to a relevant query. That context comes from the alt text, the file name, the surrounding text, and the overall page topic. Strip out any one of those elements and you reduce the probability of appearing for image search queries that could be sending you qualified traffic.

I have audited sites with thousands of product images carrying no alt text at all. The SEO impact is predictable: near-zero image search visibility, missed reinforcement of page-level relevance signals, and an accessibility failure that also creates legal exposure in some markets. The fix is not complicated. The discipline to apply it consistently is the harder part.

What Does Good Alt Text Actually Look Like?

Good alt text is specific, accurate, and written in plain language. It describes what is in the image in a way that would be useful to someone who cannot see it. That framing is deliberate, because it keeps you focused on description rather than keyword insertion.

Consider a product image for a navy blue leather wallet. Poor alt text looks like this: “wallet leather buy wallet men’s wallet navy wallet.” That is keyword stuffing, and it fails both the accessibility test and the SEO test. Google’s guidelines explicitly flag keyword stuffing in alt text as a spam signal, and screen reader users hear that string read aloud verbatim.

Better alt text for the same image: “Navy blue leather bifold wallet with card slots.” That is descriptive, accurate, and naturally includes the terms a user might search for without forcing them in artificially. If the brand name is relevant and visible in the image, you can include it. If it is not visible, do not invent reasons to include it.

The length question comes up regularly. There is no hard character limit enforced by Google, but most practitioners treat 125 characters as a practical ceiling, partly because that is where many screen readers truncate. In practice, most useful alt text falls between 50 and 100 characters. If you find yourself writing more than that, you are probably describing context that belongs in the caption or body copy rather than the alt attribute.

One thing I noticed when working across e-commerce clients managing large product catalogues: the teams that wrote alt text as part of their product data workflow, rather than as a post-publication SEO task, produced consistently better output. When alt text is written by someone who understands the product, it is more accurate. When it is written by someone running an SEO audit six months later, it tends to be generic. Process matters here as much as technique.

When Should Alt Text Be Left Empty?

Not every image needs alt text, and understanding when to leave it empty is as important as knowing how to write it well. Decorative images, those that serve a purely visual or aesthetic purpose and add no informational value, should carry an empty alt attribute rather than a descriptive one. The correct markup is alt=”” rather than omitting the attribute entirely.

The distinction matters for both accessibility and SEO. If a screen reader encounters an image with no alt attribute at all, it may read out the file name instead, which is often a string of numbers or an auto-generated identifier. An empty alt attribute signals to the screen reader that the image can be safely ignored, which is the correct behaviour for decorative content.

For SEO purposes, an empty alt attribute on a decorative image is neutral. It does not help or hurt. What hurts is using filler text like “image” or “photo” or repeating the page title in the alt attribute of every image on the page. That creates noise rather than signal, and it dilutes the relevance of the alt text that actually carries useful information.

Logos, icons, and interface elements used in navigation typically fall into the decorative category unless they carry specific informational content. A company logo in the header is decorative in the context of a blog post. A diagram explaining a technical process is not decorative, and it deserves thorough alt text even if writing it takes more effort.

How Does Alt Text Fit Into Image Search Strategy?

Image search is undervalued as a traffic channel in most SEO strategies. The majority of attention goes to the main web index, which makes sense given where most clicks originate. But in specific verticals, image search drives a disproportionate share of discovery, and ignoring it is a competitive gap you are voluntarily creating.

E-commerce is the obvious example. When someone searches for a specific product, the image carousel in Google’s results often appears above the organic listings. Appearing there requires your images to be indexed correctly, which requires alt text that accurately describes the product, supported by structured data where relevant. The teams that optimise for image search in competitive product categories tend to see meaningful incremental traffic that does not show up clearly in standard rank tracking because image search queries are tracked differently.

Real estate is another vertical where image search matters considerably. Tools like Ahrefs’ SEO resources for real estate professionals highlight how property imagery drives search behaviour, and alt text on listing photos is a basic requirement for image indexation in that context.

Beyond those verticals, image search matters for editorial content, recipes, travel, fashion, and any category where visual discovery is part of the purchase or research experience. If your business operates in one of these areas and you have not audited your image alt text, you are measuring performance against a ceiling you have artificially lowered.

I spent several years working with clients in the publishing space, and one pattern was consistent: sites that treated image SEO as a separate workstream from content SEO consistently underperformed against sites that integrated both. The images were often the entry point for a session that converted. Treating them as decoration was a measurable mistake.

What Role Does File Name Play Alongside Alt Text?

Alt text does not operate in isolation. Google uses multiple signals to understand an image, and the file name is one of them. An image saved as DSC_00847.jpg tells a crawler nothing. An image saved as navy-blue-leather-bifold-wallet.jpg tells it quite a lot, and that information works in concert with the alt text to build a coherent picture of what the image shows.

The file naming convention is easy to implement at source and almost never done correctly by default. Camera software, CMS uploads, and stock image libraries all tend to produce file names that are either auto-generated strings or generic descriptors. Building a naming convention into your image workflow before upload is significantly easier than renaming files after the fact, and it is one of those small disciplines that compounds over time.

The surrounding body copy matters too. Google’s documentation is explicit that it uses the text around an image to help understand its context. An image of a bifold wallet sitting inside a product description page about bifold wallets will be indexed with more confidence than the same image sitting on a page with sparse or unrelated text. This is why alt text optimisation in isolation, without attention to the broader page content, tends to produce modest results. The signals need to be consistent and mutually reinforcing.

Caption text is a separate element from alt text, but it contributes to the same contextual picture. Captions are visible to users and crawled as body text. Where captions are appropriate, they add a layer of context that neither the alt text nor the file name can fully provide on their own.

How Should You Approach Alt Text for Complex Images?

Infographics, charts, diagrams, and data visualisations present a specific challenge. The content within them is often the most valuable information on the page, but it cannot be conveyed in a single alt text string without becoming unwieldy. There are a few ways to handle this.

The first is to provide a brief alt text that identifies the type of image and its core subject, then include the full data or explanation in the body copy or a linked transcript. For example, alt text for a bar chart showing quarterly revenue growth might read: “Bar chart showing quarterly revenue growth across four business units, 2023 to 2025.” The underlying data lives in the text of the page, which is where it should be for SEO purposes anyway.

The second approach, more common in accessibility-focused implementations, uses the longdesc attribute or an aria-describedby reference to point to a longer description elsewhere on the page or in a linked document. This is technically sound but rarely implemented in practice outside organisations with mature accessibility programmes.

From a purely commercial SEO standpoint, the most effective approach for complex images is to ensure the key information is available as text on the page. Search engines index text reliably. They index the content of images less reliably. If your infographic contains data that supports your page’s topical authority, that data should exist in text form somewhere on the page, not only as pixels in an image file.

This is a point I have made repeatedly when reviewing content strategies for clients who invested heavily in infographic production. The visual assets were strong. The problem was that the information inside them was invisible to crawlers because no one had transcribed the key points into body copy. The SEO value of the content was being left entirely on the table.

How Do You Audit Existing Alt Text at Scale?

If you are working with a site that has been running for several years, you are almost certainly dealing with a backlog of images with missing, duplicate, or poorly written alt text. Auditing this at scale requires a systematic approach rather than a page-by-page review.

Most technical SEO crawlers will extract alt text data as part of a site crawl. Screaming Frog is the most commonly used tool for this, but any crawler that exports image data will do the job. The output you are looking for is a list of images segmented by: missing alt text, empty alt text, alt text that matches the file name, alt text that exceeds practical length limits, and duplicate alt text across multiple images.

Prioritise by page value. Images on high-traffic pages, key product pages, and pages targeting competitive queries should be addressed first. Images on low-traffic pages from several years ago can wait. Treating every image on the site as equally urgent is how alt text audits turn into projects that never get finished.

For large e-commerce sites with tens of thousands of product images, manual alt text writing is not realistic. The practical solution is to build alt text generation into the product data model, using product name, colour, material, and category fields to construct alt text programmatically. This produces consistent, accurate alt text at scale without requiring manual intervention for every image. It is not as precise as hand-written alt text, but it is substantially better than blank attributes across an entire catalogue.

Tools like Moz’s site reporting tools can help you understand the broader on-page health of your site alongside image-specific issues, giving you a more complete picture of where alt text problems sit within your overall SEO priorities. Similarly, running structured experiments on-page, as outlined in resources like Rand Fishkin’s SEO health experiments, can help you validate whether your alt text changes are producing measurable results rather than assuming they are.

Does Alt Text Directly Influence Page Rankings?

This is the question that generates the most debate, and the honest answer is: it depends on what you mean by “directly.” Alt text is not a primary ranking factor in the way that content quality, backlinks, or page experience signals are. It is a supporting signal, and supporting signals matter most at the margin, where two pages are otherwise closely matched on the primary factors.

What alt text does reliably is reinforce topical relevance. A page about kitchen knife sharpening that includes images with alt text describing knife sharpening techniques sends a consistent signal about its subject matter. A page with the same content but blank alt text sends a slightly less coherent signal. In a competitive query environment, that difference can matter.

The more direct impact is on image search rankings, where alt text is a primary factor rather than a supporting one. If image search is relevant to your traffic model, alt text moves from a marginal consideration to a core optimisation task.

I have seen this pattern play out in competitive verticals where the difference between first and fifth in image search was almost entirely attributable to image optimisation quality. The underlying page content was comparable. The structured data was comparable. The alt text and file naming were not. The sites that had invested in image SEO were capturing a meaningful share of discovery traffic that their competitors were not even measuring.

This connects to a broader point about how performance gets assessed. If you are only tracking organic web rankings and ignoring image search, you are measuring a subset of your actual search visibility. The sites that appear to be performing similarly in web rankings may be diverging significantly in total search presence. Context changes the picture entirely, and that is worth keeping in mind when evaluating where alt text sits in your priority list.

Experiments on text-level SEO signals, including work covered by Semrush’s SEO split testing research, reinforce the general principle that on-page signals interact with each other. Optimising one element in isolation rarely produces dramatic results. Consistent optimisation across multiple elements, including alt text, tends to compound.

What Are the Most Common Alt Text Mistakes?

The most common mistake is leaving alt text blank. This is partly a workflow problem and partly a prioritisation problem. When images are uploaded quickly and publishing velocity is the priority, alt text is the field that gets skipped. The cumulative effect across a large site is significant.

The second most common mistake is keyword stuffing. This tends to happen when someone with SEO responsibility but limited editorial judgment writes the alt text. The instinct is to include as many relevant terms as possible, which produces alt text that reads like a tag cloud rather than a description. Google’s systems are good at identifying this, and it tends to produce worse outcomes than straightforward descriptive text.

Using the image file name as the alt text is a third common failure, usually the result of a CMS auto-populating the field with whatever the file is called. If your file names are descriptive, this is less damaging. If they are auto-generated strings, it is worse than leaving the field empty.

Duplicate alt text across multiple images is a subtler problem. If every product image on a category page carries identical alt text, the signal value is diluted and it creates accessibility confusion for screen reader users handling the page. Each image should be described individually, even if the differences are minor.

Finally, using “image of” or “photo of” as a prefix is unnecessary. Screen readers already identify the element as an image. Starting with “image of a navy blue wallet” is redundant. Start with the description directly.

Alt text is one piece of a larger SEO system. If you want to see how it connects to the broader set of decisions that determine search performance, the Complete SEO Strategy hub covers the full range of factors that move rankings, from technical signals through to content quality and authority building.

About the Author

Keith Lacy is a marketing strategist and former agency CEO with 20+ years of experience across agency leadership, performance marketing, and commercial strategy. He writes The Marketing Juice to cut through the noise and share what works.

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