Anything on a page that isn’t text — a photo, an icon, a chart, a button drawn as an image — needs a text version that does the same job. Text can be read aloud by a screen reader, shown on a braille display, enlarged or translated; a picture can’t do any of that on its own.
This guideline has one success criterion, 1.1.1 Non-text content: what a good alternative says, and when an image is decorative and should say nothing at all.