D.B.
Go back and read what you've said in prior posts about your daughter. You've been very concerned about the people she associates with and what kinds of things they do or want to do. You say how sweet and wonderful she is but you worry about these other temptations and influences.
Now, you post this sort of idealized view of how sweet and wonderful she is - without taking into account the reality that kids have many aspects to their personality! She's not a little girl, and she's not a mature adult. She's somewhere in between. Like every other teen.
So, for over a year, she has been exposed to things you just can't imagine. And, like most teens, she hasn't told her parents everything. If you thought she was telling you everything, you wouldn't have been checking her phone! I think you were right to check her phone, and I'm sorry you found something.
But you have got to get past being heartbroken and vulnerable, and you have to be the grown-up here. If it were me, I'd get some objective legal advice. That might be from an attorney, and it might be from the police - they have trained officers to deal with this, and you will not be the first parent whose kid has done something dumb. And no, don't assume that they're going to indict her! The photo is out there. It's not going away. You will gain nothing by failing to confront this issue. You need to protect her from here on out, and that means some honest conversations. But I don't think you're ready to talk to her - your thoughts are a huge jumble and you are still in the "I'm hurt" mode. You have got to get out of that place and into a parental responsibility mode.
Yes, you've talked to her about this, but she did it anyway. That's the same feeling of every parent whose kid has sex, does drugs, drinks, drinks/drives, forces himself on a girl, bullies someone, shoplifts from a store, steals money from Grandma's purse, or gets hold of a weapon. Yes, it's awful. But it's not unique, and we all have to step up to the plate and let our kids know that yes, there are going to be repercussions. Your daughter sent a naked photo and somehow believed a guy that he would delete it or keep it private. So she was naive and dumb. That's where you start.
Get some help for this if you don't think you have the resolve. You absolutely MUST get past this idea of a perfect and sweet and innocent child. There's no such thing, so you must stop perpetuating that image. And if there's a part of you that thinks you failed as a parent, then you have to confront that as well. Get some short term counseling so you have someone realistic, experienced and objective to talk to.
But do not ignore this, and do not have a big crying fit with your child about how much she disappointed you. You have to model the maturity you want her to emulate. That means telling her that parenting is hard work, it involves "trust but verify" and checking her phone because you have to protect her.