Should I bring it up? Or leave it be??

I usually don’t go through my boyfriends phone. But i just happened to glance over at his phone while he was on twitter. I saw his recent pictures.

(For those who don’t have twitter or want an example here it is)

So! Like I said. I glanced over and saw a nude women. She was a lady from porn I’m guessing because of how the photo was set up. And I’ve always asked him was he into that type of stuff and he will always deny it. I’ve even offered to watch porn with him so we can learn a couple more moves ;) anyways.

I was wondering if I should leave it alone because “that’s what guys do” or should I ask him what was he doing with that picture...