Man, that's rough and I'm sorry to hear it. My guess would be- gf told the guy she has a BF and can't see him seriously/regularly, and the guy wanted to take his ball and go home? Pics are tough though, I can only imagine the kind of blind rage you felt.
I would assume the pics are incriminating and graphic? That sucks terribly (no pun intended) and if you can't see yourself in forgiving your gf or continuing with the relationship with the pictures permanently etched in your brain then best to move on. Good luck man.