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.
Some clients can’t handle the new lemmy feature for caching images on the instance. Hopefully its resolved soon.