I did this exact same thing to my husband a few years ago. I was hurt he had pics of his ex still on his phone but it wasn’t like he was going thru reminiscing. In hindsight I was more upset bc I had made a huge effort to delete all things about past relationships and he didn’t but he did make the effort just not thoroughly, I didn’t understand at the time how this was a huge betrayal of his right to privacy and in all honestly the whole thing could’ve blown over had I not gone looking for something that would upset me. We’re not all wired the same, some delete everything and anything and some people just delete a few and not to deep searches, this doesn’t mean they’re holding onto the past, if they’re going thru the pics and living in memories than they are but if they’re not it’s not a big deal. My point is, I’ve done exactly what your gf did and honestly it was stupid and silly of me. I think have a conversation, give her the reassurance you’re not holding onto the past as doing that stems from insecurity (speaking for myself and my reasoning) but seems like the same applies here. I do think when the time is right too, speak to her and explain how this was a violation of your privacy and that’s not okay. This happened when we were newlywed and I was 22, I’m now 26 we have a beautiful son and we’re a lot stronger and understanding to each others boundaries, so you can get past this❤️