Usage
Alternative text or labels for images must describe the meaning of the image.
Is this image decorative or informative?
If the image conveys important meaning and no other text on the page explains the content within it, then the image is likely informative.
If the image is included for purely stylistic purposes and doesn’t impart any meaning to the rest of the content on the page, then use decorative image.
If your image contains meaningful text inside it, it should not! This is a violation of WCAG AA 1.4.5 Images of Text. Exceptions exist for elements like logos.
How to write alternative text
If you were describing the image to someone via phone conversation and they couldn’t see what you were looking at, what would you say?