This is unironically a good guide on how to write alt-text that would be good for fedi users to read:
https://accessibility.huit.harvard.edu/describe-content-images
Write helpful Alt Text to describe images
Alternative (Alt) Text is meant to convey the “why” of the image as it relates to the content of a document or webpage. It is read aloud to users by screen reader software, and it is indexed by search engines.accessibility.huit.harvard.edu