When using the in-app share button to share images, a file called "image_proxy" is shared, not the image itself. This started happening for me when I switched to lemmy.zip from lemm.ee - I just tested lemmy.zip in Thunder and it works as expected, so seems to be some combination of server+Boost causing the problem.
Yep, I've been having the same problem for a while now. I've seen other posts about it here and there, but I'm not sure how actively boost is actually being developed...
I'm currently fixing this issue