Depends on the relationship that you have with him / her. If your "Lover" is just a friend with benies then you don't have much to say about it. If your "Lover" lives in your home and you share the bills and all then yes you can say, Hey what's up with this? If your "Lover" is your spouse then you really have a right to say something.
It depends on how you feel about it. If you don't care then ignore.. if it bothers you then say something. You haven't said how you feel about it. Some people in certain types of relationships aren't threatened by stuff like that, but if you feel its a betrayal of trust or dishonest then I'd say confront. Me and my wife have an agreement, which hasn't always been followed... that if there is the slightest thought of that the other person might not be ok with something, then that is when to talk before doing it. For the sake of the relationship.
you let him catch you looking at **** pics sent to you by other men.
Why waste time confronting him? It's unlikely he'll change. Dump him.
Confront, of course.
I'd say you'd have to confront. That means his mind isn't on you. He's fantasizing about other women. He can't be all the way there for you if he's fooling around with them like that.