Should I bring this issue up?

My guy and I have been together for two years and four months. He proposed in December. We've got a pretty good relationship. I was looking for pictures of us on his computer one day and also looking for old im's we used to send each other. I found a messenger folder with the title "just show me your t*ts." In it were two copies of a picture of his "manhood." This was nothing he'd sent me before. The first picture was created before we started dating. The copy was added after we'd been dating a year. These pictures appear to be default pictures for an im'ing session. We use to talk on that particular messenger, but only a little bit and only when we had just started dating. And we didn't trade pictures or anything on that particular messenger. So what does this mean? Was he "talking" to someone else online? He no longer has this messenger installed. Was it a moment of weakness? Does it mean he even did anything? Should I even discuss this with him? I don't want to start something over something petty. Am I just coming off like I don't trust him? I pretty much just want to forget about it and focus on the positive things in our relationship. Does that sound like the best thing to do?