Uploading GIFs on this site has been hit-or-miss for the few months I've been here, but where they used to fail with an error page a while back, the GIF is now quietly discarded from the thread. Posting GIFs to this instance is broken from kbin.social as well (where the whole post fails) so it's not a front-end issue.
@jerry@fedia.io They range from 2.7 to 9.1 MB, though looking at the ones that did post successfully, they seem to be less than 5 MB. I think I ran into a size limit.