What is Image Alt Text?
Image alt text is a short written description of an image, set on the HTML alt attribute (for example, alt="Black leather Chelsea boot, side profile"). It is invisible on the page but read aloud by screen readers, shown when an image fails to load, and parsed by AI engines and search crawlers. WCAG 2.1 Success Criterion 1.1.1 requires a text alternative for all non-text content — meaning every meaningful image needs alt text.
AI engines like ChatGPT, Perplexity, and Google AI Overviews increasingly cite multi-modal content. Most do not run vision on every image — they read the alt text, captions, and surrounding HTML to decide what an image shows and whether to cite the page. Pages where text, images, and structured data all reinforce the same topic get cited far more often. This metric is part of the Technical Foundation pillar in your GEO-Score.
Why Alt Text Matters for AI Search
Alt text is one of the few signals that pulls triple duty: it is a legal accessibility requirement, a confirmed image search ranking factor, and the primary text input AI engines use to interpret your visuals. Skipping it costs you on all three fronts simultaneously.
Accessibility is the baseline (and the law)
WCAG 2.1 Level A requires a text alternative for all non-text content. The WebAIM Million 2025 study found 18.5% of all home page images still lack alt text — and missing alt text is the most common WCAG violation cited in the 3,117 ADA Title III website accessibility lawsuits filed in 2025.
Alt text is how machines read your images
Google says it uses alt text, together with computer vision and the rest of the page, to understand what an image shows. AI tools that read your HTML get the same clue from the alt attribute, the caption and the text around the image. Without good alt text, your images say very little to them.
Image search and visual citations need text
Google Lens processes more than 12 billion visual queries per month and is growing 30% per year. Descriptive alt text is the single highest-impact image SEO action — but a Backlinko study of 65,388 visual searches found only 11.4% of ranking images had alt text matching the query.