Alt text
Alt text is a short written description of an image, read out by screen readers and used by search engines and bots to understand it.
Alt text is a short written description of an image, added to the image's HTML in the alt attribute. A screen reader reads it aloud to people who can't see the picture, and browsers show it when the image fails to load. Search engines and other bots read it too, as one of the ways they work out what the image shows.
For a marketer, alt text is a small job with a long reach: it makes your pages usable for more people, it helps your images appear in image search, and it puts into words what would otherwise be locked inside a picture.
Writing good alt text
Describe what the image shows and why it's on the page, in a sentence or less. There's no need to begin with "image of" or "picture of" before the description. If the image is pure decoration, leave the alt text empty so screen readers pass over it.
Google's image guidelines ask for alt text that's useful and fits the page, and warn against stuffing it with keywords. They also note that when an image is a link, its alt text works like the link's anchor text.
For example, on Harrow & Pine's product page:
<img src="oak-table.jpg" alt="Harrow & Pine oak dining table, seats six, natural oil finish">That says more than alt="table", and far more than alt="oak table dining table best oak table buy".
Alt text and AI search
A bot that reads your HTML gets the alt text, not the picture. So any fact that lives only in an image, like a size chart, a price list or a product feature, is invisible to it unless you put it in words.
Alt text helps, but it's short by design. If an image holds information people ask about, put that information in the page's text as well, where it can be read, quoted and cited.