Well I guess this depends on your interpretation of the age old question: what constitutes cheating?
From where I'm sitting, he's probably cheating in some fashion because he's not discouraging his ex-wife from sending naked pictures. The fact that he and his ex-wife are even still that close to be cavorting as such gives me pause. In fact, I'd assume they were still smanging on occasion given the nudey pics. Most folks who are ex's by law tend to not even want to speak, much less see each other in public. So pictures? Yeah, I'd assume something was going on.
But let's say that she's just sending him pictures and he's not acting on it at all. If you only think cheating is sleeping with somebody else, then no, he's not cheating. He's definitely not absolved from wrong-doing. Unless you know for a fact that he's told her to stop sending pictures.
Just receiving pictures doesn't constitute cheating. What if some chick just randomly sends him some nudey pics. That's not his fault. However if he starts receiving lots of pics, then my guess is that somewhere he told her it was okay, which in my book, constitutes cheateration. Throw something at him.
Now...pictures do not guarantee that he's actually going out there sleeping with his ex, but I wouldn't be surprised. So it's not proof, but its definitely reasonable suspicion. Again, throw something at him before you ask him why he's getting pics of his ex-wife.
Oh yeah, that's a form of cheating. No one should be receiving any nudie pics from anyone if they are in a committed relationship, except from their partner. In of itself, this is not evidence of a physical affair, though certainly enough to raise eyebrows and is a major trust issue, especially if he did not tell you and you found out another way. You have a right to wonder what else is he doing behind your back.
" No one should be receiving any nudie pics from anyone if they are in a committed relationship..." That's a tricky statement to make, because while you're not wrong, as Panama alluded to, if someone is receiving unsolicited nudie pics, they can't help that. Yes, depending on how serious the situation is one could probably get a restraining order or block the ex's number, but if they have kids together or something that may not be a viable option at this time. Some people send out nudie pics like hotcakes, and there's not much you can do as the receiver except delete, and ask them not to do it again, though I've known these people to completely forget a request like that once they are no longer sober. He is very probably doing something wrong, but I think the first step here is finding out in what context these pictures are being received. Because as likely as it is that this guy is cheating, it's just as likely that the question asker happened to see a message pop up on the coffee table that was sent while her s.o.'s ex was drunk and depressed. We don't know.
I see your point. But...the nature and wording of the question does not suggest he's been getting them unsolicited, or if he has been, it don't sound like he's put much effort into putting a stop to it. It's one thing to get an unsolicited nude picture. You don't want it, delete it, tell the person to stop. If he's been doing that, I doubt the OP would have asked this question, instead, she'd probably ask something along the lines of "How do I get my BF's ex-wife to stop sending nude pictures".
Since this wasn't asked, I'm gonna have to go along with he's going along with the reception of nudes.
It almost seems like your BF is trying to make you squirm. He should have told his ex to stop sending the pics and not even said anything to you. Is he trying to let you know that "doing" his ex is some kind of option if one night you don't "put out"? This type of behavior does not belong in a "healthy" relationship. His ex needs to move on with her life and quit antagonizing your BF AND you. I doubt I could put up with that for 1 second.