[Rivière]

As much as it's his phone, I don't think he realises the severity of keeping pictures of girls pictures. If they are pictures of girls underage and/or if he keeps them without their permission he could be in serious trouble.

Secondly, I don't think you're making a big deal out of it at all! If I was with someone who kept naked pictures of other girls on their phone I'd be thinking "What the hell!?" too! If he really loves and cares about you, why should he have the need to keep them? Maybe it would be good for you to ask him to be honest with you and ask him why he really keeps them there. I know some guys keep naked pictures as masturbation material but I don't think, or at least in my opinion, it's the done thing to keep naked pictures of other girls on a phone unless the person you're in a relationship with is ok with it, which you clearly aren't. I think here you need to confront him and ask him to be truthful, good or bad, why he keeps those pictures on his phone and explain to him you find it uncomfortable that he keeps them there when he's in a relationship with you.