How should I handle this?

I found an sd card outside on the ground. When I looked at what was on it I found out it was from my current live in boyfriend of a years old phone. I found a lot of pictures of a girl in a separate folder. She was making y faces, putting her finger in her mouth, lots of , tattoos near her pelvic bone. I wasn't upset until I checked the date on them. They were from when we were dating. We dated for five months broke up for five. We've been together now for a year not including the first time. We've had other issues of him getting promiscuous pics from other females and hiding them from me. He's also lied to go hang out with them without me. Things are doing better now. Well, they were until I found this. I would have just let it go but they were while we were dating. I don't even know who she is. I've never seen her and he's never told me about anyone like her.