Alt Text and SEO: Writing Image Descriptions That Actually Help

Alt text (the alt attribute on an <img> tag) does more work than most sites give it credit for. It's the description a screen reader announces to someone who can't see the image, the text Google Images uses to understand and rank a picture, and - increasingly - part of what tells an AI crawler what's actually on your page when it can't "see" images the way a person can. Get it right and it quietly helps three different audiences at once. Get it wrong (or skip it entirely) and you lose all three.

What Alt Text Is actually For

Accessibility. Screen reader software reads alt text aloud in place of the image, so a blind or low-vision visitor gets the same information a sighted visitor gets by looking at the picture. This is the primary purpose - alt text was built for accessibility first.
Image search. Google can't reliably interpret pixels the way it interprets text. Alt text (along with the surrounding context and filename) is one of the main signals it uses to understand what an image shows and to rank it in Google Images.
Fallback content. If an image fails to load - slow connection, broken file path, an ad blocker catching something it shouldn't - alt text is what displays in its place.
AI crawlers. Tools like ChatGPT and Perplexity that summarise or cite web content generally can't process the image itself either. Good alt text is part of what lets an AI-generated answer accurately describe something your page shows visually. See our AI crawler accessibility checklist for the wider picture on AI bots and your site.

How to Write Good Alt Text

Describe what's actually in the image, in plain language. "Plumber fixing a leaking kitchen tap" beats "plumbing123.jpg" or nothing at all.
Keep it concise - a sentence, not a paragraph. Screen readers read it in full every time, so a bloated description gets tedious fast for the people it's meant to help most.
Don't start with "image of" or "picture of." Screen readers already announce that it's an image; adding it in the text is redundant.
Include a relevant keyword only if it genuinely describes the image - never stuff keywords into alt text that don't match what's shown. That helps neither accessibility nor search rankings, and reads as spam to both.
Leave alt="" (empty, not missing) for purely decorative images - a background flourish or divider with no informational content. An empty alt attribute tells screen readers to skip it silently; a missing attribute often gets read aloud as the filename instead, which is worse.

Common Alt Text Mistakes

Every image left with the default filename - IMG_4821.jpg read aloud to a screen reader user, and meaningless to Google Images.
The same alt text copy-pasted across every product photo - "product photo" repeated forty times helps no one and looks like low-effort content at scale.
Alt text stuffed with keywords instead of a description - "plumber London emergency plumber cheap plumber near me" isn't a description of anything; it's a red flag to both screen reader users and search engines.
Decorative images with no alt attribute at all, rather than an explicit empty one - this is a common accessibility failure that's easy to fix once you know the difference.

Where This Fits in a Wider Audit

Alt text is one check among many that determine whether a site is genuinely accessible and crawlable - alongside heading structure, colour contrast, and keyboard navigation. See our guide on what website accessibility actually means for the fuller picture. AuditCrow's free scan flags missing and empty alt attributes automatically across every page you check, so you don't have to inspect each image by hand.

The Bottom Line

Alt text is one of the cheapest, highest-leverage fixes on most sites - it takes seconds per image and genuinely helps real visitors, not just a crawler. Write it for the person who can't see the image first; the SEO and AI-visibility benefit follows naturally from doing that well, not from treating it as a keyword slot.