@Ricardus sadly no.

what you can do is either:
- reply to a post with the same image but with alt text (might offend the original poster, especially if it's their own art)
or
- message the poster with your alt text and hope they remake their post

myself and some friends will post a "hey I need help describing an image, please message me" and work it out via dms, but I dunno if that's standard practice on mastodon, lol