Boyfriend possibly cheating? Help

So tonight I went over my boyfriend of 5 year’s apartment for dinner like we usually do. We decided on getting delivery and my bf went to open the app on his phone, but when he opened his phone I clearly saw a nude picture of a woman on a bed. I saw enough where it looked like whoever took the picture took it in a dark room like at night, so I’m sure it wasn’t some random person’s post on social media or something. He closed it really quickly and I didn’t see what app it was on or if it was in his messages, and I think he thinks I didn’t see it because I didn’t say anything at first. And then I was too chicken to mention it through dinner so I went home later without mentioning it at all.

We’ve been together for 5 years and he’s never done anything that would lead me to distrust him, and I don’t want to snoop through his phone. However, I’m really worried about this :( what should I do? Should I just not mention it or should I ask him about it the next time we see each other? I don’t want to make things awkward but idk.