Informative images convey a simple concept or information that can be expressed in a short phrase or sentence. The text alternative should convey the meaning or content that is displayed visually, which typically isn’t a literal description of the image.
In some situations a detailed literal description may be needed, but only when the content of the image is all or part of the conveyed information. Whether to treat an image as informative or decorative is a judgment that authors make, based on the reason for including the image on the page.