The alt-text on my instance came through as just the post title Alt-text test with Voyager (fallback behavior for when there is no alt-text specified). Looking at the api spec, it doesn't look like we have a custom alt-text field for creating a post. So, this is something that is web UI exclusive right now, but it should be relatively easy to include this in the next PieFed version.

I will make a codeberg issue for this. Edit: done