Excellent question. For image-based posts, there is currently no mechanism to directly embed alt-text. This is likely to change in the future, however, so subscribe to lemmy#1086 for updates.
For now, the general advice is to use the post body to describe the linked image (on Lemmy, all posts are allowed to have “self-post”-like text). This would work by linking the image directly as usual and also embedding the same image in the post text with alt-text included there.