You're being a dumb douche accepting pics off a random girl, that's weird... unless you were trying it on with her, which you claim, you are not.
Telling your girlfriend was silly too, if you did not want to see pics of a semi naked chick who you don't want to get it on with. Why? Because nothing truly terrible has happened, if you are genuinely innocent (and stupid at the same time). You should of simply just stopped talking to this friend of a friend, deleted the pics and never spoke of the incident again.
She is over reacting and if she's breaking up with you over that then if this incident hadn't occurred you're going to break up down the line over something as silly anyway.
I'm pretty sure she'll get back with you... Surely this cannot be the reason for a Couple in Love to break up.
Best of Luck.