Image SEO: How to Write Alt Text That Ranks

If you’ve ever uploaded an image to your website and left the alt text field blank, you’re not alone — but you’re also leaving real SEO value on the table. Alt text is one of the most underestimated tools in image SEO, and it does far more than most people realise. It helps search engines understand your visuals, improves your site’s accessibility, and can contribute meaningfully to how your pages rank in both standard and image search results.

This guide breaks down everything you need to know about writing good alt text — what it is, why it matters, and how to do it properly without making your content feel robotic or over-optimised.

What Is Alt Text and Why Does It Exist?

Alt text (short for alternative text) is an HTML attribute added to image tags. It describes what an image shows, and it was originally created for accessibility purposes — specifically to help visually impaired users who rely on screen readers to navigate the web.

When a screen reader encounters an image, it reads out the alt text aloud. Without it, the user hears something useless like "image" or "graphic" — or nothing at all. For anyone relying on assistive technology, a website full of undescribed images is genuinely difficult to use.

Beyond accessibility, alt text became a critical part of image SEO because search engine crawlers can’t "see" images the way humans do. They read the alt attribute to understand what the image depicts, which helps them index it correctly and determine its relevance to the surrounding content.

How Alt Text Influences Image SEO

It Tells Google What Your Image Is About

Search engines like Google use alt text as one of the primary signals when indexing images. Without it, Google is essentially guessing — relying on surrounding text, file names, and page context to figure out what your image shows.

A well-written alt attribute removes the ambiguity. If you have a photo of a hand-thrown ceramic mug on a product page, an alt text like alt="hand-thrown ceramic mug in blue glaze" gives Google a clear, accurate description to work with. That image then has a much better chance of appearing in relevant image search results.

Image Search Is a Real Traffic Source

Google Images accounts for roughly 22.6% of all web searches, according to data from SparkToro. That’s not a niche channel — it’s a significant portion of search activity that many businesses are completely ignoring.

Optimising your images with descriptive alt text can help you appear in image results for queries you might not currently rank for in regular web search. For e-commerce sites in particular, this can directly influence purchase decisions, since many shoppers use Google Images to browse products before clicking through.

Accessibility and SEO Are Aligned

One of the more interesting things about image SEO is that doing it right for people and doing it right for search engines are essentially the same task. Writing accurate, descriptive alt text benefits screen reader users and search crawlers equally.

This alignment is worth noting because it means you shouldn’t have to choose between accessibility compliance and SEO performance. Writing genuine, human-facing descriptions is the right move on both fronts.

Writing Alt Text That Actually Works

Be Descriptive, Not Keyword-Heavy

A common mistake is treating alt text as another place to stuff a target keyword. That approach is both ineffective and counterproductive.

Google’s own documentation advises writing alt text that’s useful and informative — not text that’s "full of keywords." If you’re optimising a page about "Dublin city breaks" and you have a photo of Trinity College, the alt text alt="Dublin city breaks Trinity College Dublin tourist attraction holiday" reads as spam. Something like alt="Trinity College Dublin's famous Book of Kells library entrance" is more descriptive, more natural, and ultimately more useful.

Keep It Concise

There’s no official character limit for alt text, but a practical guideline used by many accessibility experts is to keep it under 125 characters. Screen readers may cut off descriptions beyond that point, and overly long alt text tends to become vague or unfocused anyway.

Aim for a single, clear sentence that describes the image as if you were explaining it to someone who can’t see the page. That mental framing tends to produce better alt text than trying to optimise it.

When to Leave Alt Text Empty

Not every image needs alt text. Decorative images — things like background textures, dividers, or icons that exist purely for visual styling — can use an empty alt attribute (alt=""). This tells screen readers to skip the image entirely, which is actually the correct accessible behaviour.

The distinction matters. If an image conveys information or is relevant to the content around it, it needs descriptive alt text. If it’s purely ornamental, leave the attribute empty rather than forcing a meaningless description.

Common Alt Text Mistakes to Avoid

Even people who understand alt text in principle often make the same avoidable errors. Here are the most frequent ones:

Starting with "image of" or "photo of": Screen readers already announce that it’s an image. Saying "image of a coffee cup" is redundant — just say "coffee cup on a wooden desk."

Using the file name as the alt text: alt="IMG_4823.jpg" is meaningless and unhelpful.

Writing the same alt text for multiple images: If you have five product photos, each one likely shows something slightly different. Describe each individually.

Over-describing background elements: Focus on what’s relevant to the content, not every visual detail in the frame.

These mistakes are easy to slip into, especially if alt text is being added quickly or in bulk. Slowing down and thinking about what each image actually communicates will produce noticeably better results.

Alt Text in Practice: Industry-Specific Considerations

E-Commerce Sites

For online retailers, alt text is particularly high-stakes. Product images often appear in Google Shopping results and Google Images, both of which drive purchase-ready traffic.

Best practice for product images is to include the product name, a key descriptive detail (colour, material, size if relevant), and optionally the brand name. For example: alt="women's leather ankle boots in tan brown with block heel". This mirrors how users actually search for products.

Blog and Editorial Content

In editorial contexts, images often support a story or illustrate a concept. Alt text here should reflect that role — describing the image in a way that makes sense in the context of the article.

If you’re writing about urban farming and include a photo of raised vegetable beds on a rooftop, alt="raised vegetable beds on a Dublin rooftop garden" connects the image to the topic and the location in a natural way.

Local Business Websites

For businesses serving a specific area, adding relevant location context to alt text (where it’s accurate and natural) can support local SEO. A plumber in Cork who uses a photo of a bathroom renovation they completed could write alt="modern bathroom renovation in Cork with walk-in shower". It’s truthful, descriptive, and locally relevant.

Don’t force location into every image description — only where it genuinely adds meaning.

Technical Basics: Getting the HTML Right

Alt text sits inside the <img> tag in HTML, like this:

If you’re using a CMS like WordPress, Squarespace, or Shopify, there’s usually a dedicated alt text field when you upload or edit an image — no coding required. Most platforms make it straightforward, but many users either skip the field or leave auto-generated placeholder text in place.

It’s worth auditing your existing images periodically. Tools like Screaming Frog, Ahrefs Site Audit, or even Google Search Console can flag images with missing or empty alt attributes, giving you a clear starting point for improvement.

How to Audit and Improve Your Existing Alt Text

If you’ve never reviewed your site’s image alt text, here’s a simple process to get started:

Crawl your site using a tool like Screaming Frog (free up to 500 URLs). Export the image data and filter for missing alt text.

Prioritise high-traffic pages first. Check which pages get the most visits via Google Analytics and start there.

Review images in context. Look at each image alongside the surrounding content — what role does it play? Write alt text that reflects that role.

Check for duplication. If multiple images share the same alt text, update them individually.

Test with a screen reader. Tools like NVDA (free for Windows) or VoiceOver (built into Mac and iOS) let you experience your site the way a visually impaired user would.

This isn’t a one-time job. As you add new content and images, building alt text into your publishing workflow is the most efficient long-term approach.

Frequently Asked Questions

What is alt text in simple terms?
Alt text is a short written description added to an image on a web page. It tells both screen readers and search engines what the image shows. It appears in the HTML code but isn’t visible on the page itself — unless the image fails to load.

Does alt text really affect my Google rankings?
Yes, though it’s one signal among many. Alt text helps Google understand and index your images correctly, which can improve your visibility in both image search and regular web search. For image-heavy sites like e-commerce stores, it can have a noticeable impact on traffic.

How long should alt text be?
Most accessibility guidelines recommend keeping alt text under 125 characters. It should be long enough to be descriptive and accurate, but short enough to stay focused. A single clear sentence usually does the job.

Is it worth adding alt text to every image on my site?
You should add meaningful alt text to every image that conveys information or is relevant to the content. Purely decorative images (backgrounds, dividers, icons) should have an empty alt attribute (alt="") rather than a forced description.

Can I use my main keyword in alt text?
Yes, if it’s natural and accurate. If your keyword genuinely describes what’s in the image, using it in the alt text makes sense. The key is that it should flow naturally — not be inserted just for SEO purposes. Google can detect over-optimisation, and it can work against you.

Conclusion

Alt text might seem like a minor detail, but it sits at the intersection of accessibility, user experience, and image SEO — three things that genuinely matter for any website that wants to perform well and serve its audience properly. Writing good alt text isn’t complicated, but it does require intention and consistency.

The payoff is real: better image indexing, stronger search visibility, a more inclusive experience for all users, and a website that search engines can understand at a deeper level. Whether you’re starting fresh or auditing an existing site, improving your alt text is one of those small changes that compounds over time.

Ready to Improve Your Image SEO?

If you’d like help auditing your site’s image SEO or want to discuss a broader strategy, our team is happy to talk through your options. There’s no pressure — just a straightforward conversation about what might work for your site.

Get in touch at moc.ssobebolgobfsctd-77418d@ofni or call us on +353 1 868 2345 — whichever is easier for you.

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