Hello!
They're JPGs from 855kb - 1.82mb. I tried different ones that all worked on my sh.itjust.works account. I tried one today on my .ml account that didn't work either, not sure if that's related. Maybe an update that hasn't been pushed by all the instances yet? I donno.
JPGs around this size used to upload without an issue on this account but stopped a couple days ago. The same files gave me either of those two errors on different attempts.