For posts, I can’t do alt text for those, because there’s no guarantee the links are, or have, images. Links could go to articles, videos, anything. The only option there would be to add a second field, like link_description, which I don’t really want to do. So I think for image posts, just taking the title, and communities asking to transcribe image posts in a comment is the best thing for now.
As far as images in comments, its all markdown, so  works.
https://github.com/LemmyNet/lemmy/issues/1086 check this thread out. The TL;DR is in this comment from the developer: