What is image alt text, and does it help SEO?

Alt text (alternative text) is a short written description of an image, stored in the page’s code, that screen readers read aloud to people who cannot see the image and that search engines use to understand what the picture shows. It serves two purposes:

• Accessibility first — and in Australia, web accessibility is a reasonable expectation under the Disability Discrimination Act, not just good manners.
• SEO value, real but modest — good alt text helps your images surface in Google Image search and gives Google extra context about the page.
• Write a plain description of what the image actually shows — “red kelpie herding sheep” — not a stuffed list of keywords, which both screen-reader users and Google treat as spam.
• Decorative images that carry no information can have empty alt text.

Describe the image honestly for a person who cannot see it; the SEO benefit follows from doing exactly that.