I doubt that's way this happened. More likely, the person who implemented the "inappropriate photo"-feature wasn't fully aware of that the "Report"-functionally

I doubt that's way this happened. More likely, the person who implemented the "inappropriate photo"-feature wasn't fully aware of that the "Report"-functionally was enabled for everyone and not just your friends. However, someone had to implement the backend for listing out those photos, and they clearly didn't think of access control, so there's at least something fishy here…