I say no matter what confront him. It's a matter of how you do so that may come back to bite you or maybe not. Here's all I'm saying,... And I do understand the part where you've been cheated on and all of what he said and so forth! OK? But believe it or not, some guys (although honestly, not as many as would lead you to believe) do know some girls that they are purely in a platonic relationship (i. e. friendship) with! Even some cute looking ones! So giving him the benefit of the doubt, consider this. He goes to meet up with his friends, just as he told you, and may have had every intention of doing!
So the next thing he knows more friends and friends of friends start to pile onto the "party" and everyone is simply haveing a (non-cheating) good time! Does he even think twice when and if more friends stop by or whatever, probably not, just having some normal fun, more than expected. So if he considers this girl a friend, and nothing more, then he would be hard-pressed to keep track of worrying about walking on egg-shells to be sure nothing might come across as improper! He either allowed, asked for or in some way allowed the picture to be taken and posted somewhere that you could easily see it! He's either incredibly stupid or just didn't see it as what it looks like to you because such a thought never crossed his mind!
Now, I, of course, believe your story about how you happened across the image on Facebook (wink, wink, nod, nod!) OK, only you know that, but dishonestly rarely begets honesty. No matter if it's possible to get caught or not. So consider that anything else is none of my business!
I'm simply saying that that's is not unrealistic. So, don't approach him and say, "So did you have fun? Well then WHAT'S THIS?" *holding up a copy of the picture!* You want honesty, right? Then be honest. If you did do a little "snooping", tell him, and you have the perfect excuse, you're trying to learn to trust someone fully once again! If you found them as described then the same thing goes, but just tell him and explain that you were spooked, and what's up with the pretty girl in pic (s)? Were there many girls there? Don't start throwing accusations around, just let him fill in the blanks and if he just gives short answers ask him to tell you more.
If he's going to lie, he's going to lie no matter what you say or do, so there's no way around that. And try not to play the victim, or you'll be playing mind-games w/him! That's not-cool!