Alt text is what stands in for an image when the image is not available. Screen reader users read it, so does anyone whose images failed to load, and so do search engines.
There is one common misconception: describing every image is not the goal. Put a description on purely decorative artwork and screen reader users sit through sentences carrying no information at all. Hence two rules rather than one.
Ask first: does removing this image lose information?
On the left, use an empty alt (alt=""). An empty value is an explicit signal that the image can be skipped, and readers pass over it silently. Omitting the attribute entirely is a different thing — some tools then read out the file name instead.
Informative images carry the information, not the picture
“Bar chart image” helps nobody. Write what the chart is there to say. For a diagram, one sentence of conclusion; where the individual values matter, put the same content beside the image as a short table or list.
Icon-only buttons work differently again: the alt describes the action, not the drawing. A magnifier icon is “Search”, not “magnifying glass”. A logo that links home takes the site name, not “logo”.
The same image can need different text in different places. In a product grid the name is enough; on a detail page you describe the colour or material a buyer would actually use to decide.
Text baked into an image exists nowhere
Drawing copy into a banner or infographic is common because it gives you complete typographic freedom. The catch is that those words exist for neither search, nor assistive technology, nor translation.
The fix is straightforward. Keep the background and the decoration as an image, and lay the words on top as real text. Font scaling, multiple languages and search visibility are all solved at once — and the contrast rules from the previous part become calculable again.
Finally, do not stuff keywords into alt attributes. An unnatural sentence written for search is read aloud verbatim to screen reader users, and it is not a good signal for search engines either.
How images relate to search visibility is covered further in the SEO archive, and if you want a look at the state of images and markup across a site, the diagnostic under free tools is a reasonable first pass.
Next part
Next: focus indicators. The default ring rarely suits a design, so it gets deleted — and the moment it goes, keyboard users lose every clue about where they are.