Everyone on here is immediately screaming divorce because they are assuming that she really is cheating. But I think you shouldn't abandon all hope yet. So you found evidence of her sending a selfie to another guy who she has been talking to for awhile. I would say that this is evidence of an emotional affair, as was the Facebook messaging. This is NOT hard evidence that she is physically cheating on you. That is not to say that emotional affairs are not hurtful, but she by definition may not be cheating.

This being said, you do need to confront her and have a CALM and HONEST conversation. You need answers, and you will get them by asking and letting her explain herself. I think slamming down divorce papers and presenting all this evidence you accumulated from invading her privacy would be the worst thing possible and you are immediately going to make her feel hurt and betrayed. If you want to work through it, this is NOT the way.

I am not trying to defend her because I believe what she is doing is wrong. But from what I can tell, you love her and want this to work out. So don't assume the worst yet. I know that the trust has been lost, but have an honest conversation. Tell her that you did see the initial photo and you want to know if she is cheating. Tell her that you deserve the truth. Don't accuse her.

If she is cheating physically, that's when I say lawyer up. If she is cheating emotionally, I think you should consider couple's therapy. There is something that this affair is giving her that she isn't finding in your marriage, but that I think you can work through. I know betrayal is hard and that it feels like you might never trust her again, but if you both love each other and want to get through it you can.

And a side note, maybe it wouldn't hurt to reach out to the guy's ex girlfriend again and ask for more information. Maybe she knows more that might be helpful to you in making your decisions on how to go about this.

Best of luck.