From the developer:

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 ![alt-text](link) works.