My MS son uses some of the above, and others (discord, even google classroom) where he uploads photos (appropriate ones) so there is no way that a single adult would know all the places a photo could hide, or that it had not already been sent to friends of this girl.
I think a police report is a good idea, if only to scare the offenders to think twice before doing something like this again. If the police have reason to, they also have specialists that can search the phone and find deleted and hidden files.