Alt text is the written description you attach to an image so that people using screen readers, and machines that cannot see, understand what the image shows. Search engines and AI assistants read alt text to interpret your images, index them, and decide whether to surface them in results and answers. Good alt text is not keyword stuffing — it is a plain, accurate description of the image in context. Get it right and your images become discoverable and your pages become clearer to every engine that reads them; get it wrong and a chunk of your page is simply invisible to machines.

How machines actually "see" your images

Modern engines do apply computer vision to images, but they do not rely on it alone, and they trust your explicit description far more than a guess from pixels. When a crawler or an AI assistant encounters an img tag, the first thing it reads is the alt attribute. That text becomes the image's meaning as far as the machine is concerned. Surrounding context — the caption, the nearby paragraph, the file name, and the page heading — adds signal, but alt text is the anchor. If it is missing, the engine is left inferring from a file name like IMG_4821.jpg, which tells it nothing.

This matters more in an AI-first world because assistants often work from a text representation of your page. An image with no alt text contributes zero to that representation. A chart, a product photo, or an infographic that carries real information becomes a blank spot in how the AI understands and summarizes your content.

Writing alt text that works

The goal is to describe the image as you would to someone on the phone who cannot see it — specific, concise, and relevant to why the image is on the page. A few principles keep you on track:

Context changes the right answer. The same photo of a red bicycle might be "Red commuter bicycle with a front basket" on a product page, or "Cyclist commuting in city traffic" in an article about urban transport. Describe the image for the job it is doing on that specific page.

Where alt text meets accessibility and SEO

Alt text sits at the intersection of accessibility and discoverability, which is why it pays off twice. It is a core requirement of WCAG accessibility compliance — non-text content must have a text alternative — so writing it well serves real users who depend on screen readers. At the same time it is a ranking and interpretation signal, which is why it belongs in the same mental bucket as your meta tags: small pieces of text that tell machines what your content is about.

Alt text is only part of image optimization, though. It handles meaning; it does not handle speed. To keep those richly described images from slowing your pages down, pair good alt text with modern formats and lazy loading as covered in our guide to image optimization with WebP, AVIF, srcset and lazy loading. Meaning and performance are separate problems, and you want both solved.

A practical routine for every image

Build a simple habit into your publishing flow. For each meaningful image, ask what it shows and why it is on this page, then write one clear sentence that answers both. Give decorative images an empty alt attribute. Use descriptive, human-readable file names instead of camera defaults. And spot-check a few pages the way a machine would, reading only the text and alt attributes, to see whether your page still makes sense with the images stripped away. If it does, engines and AI assistants will understand it too.

Want to know which of your images are missing alt text or invisible to bots right now? Run the free CheckMy.site scanner to see how machines read your pages and catch the gaps before they cost you visibility.