Taxon photo changes have been recorded in the taxon history page for a few months now, so you can see who’s been changing them. You can reach out to them via message or flag the taxon and @ mention them.
There’s an FAQ about taxon photo guidelines. It’s not taxon specific, however. The issue for a default photo is, as others have said, it also gets shrunken down to a tiny icon that should be at least somewhat readable, if not completely diagnostic.