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.
You must include the word "rule" in every comment and post title on Lemmy to indicate that you accept the ToS :)