My boyfriend (20m) and me (18f) have been together for roughly 8 months. Recently I found photos of him and a girl on the girls instagram. I asked him about it and he said that they had a summer fling and that it was not serious. I was at his house one day, and he had left early for work. I wanted to surprise him so i made his bed and was going to leave an “I love you” note for him after he gets off of work. I didn’t have a pen, so I did what any rational person would do. I looked in his room to see if there was one. I looked in his nightstand, which I saw a Polaroid of him and his friend, then a framed picture of him and that same girl. I wasn’t even trying to purposely snoop, I was truly just looking for a pen. Do I confront him about it? Do I ignore it, even though it will just cause resentment down the line? Im so torn at what to do. Do I even have a right to be mad at him, since I was looking in the first place? Was I overstepping? Or would it not be overstepping if I didn’t find anything? Help me please, I don’t know what to do

TLDR: I found a framed picture of my boyfriend’s “ex” in his nightstand. He claims that they were not serious. How should I proceed?