I found an extremely NSFW, inappropriate photo on my 17 year old`s phone.

Okay I'm not exactly sure which topic I should post this in. But I am a mom of three kids, all girls. 13, 17, and 18. My daughters (17) and (18) both have boyfriends but I can be stern at times regarding where they are and what they can do. Not my older daughter because she’s technically an “adult” now but I still have some rules since she still lives with me. My middle daughter (17) and her boyfriend (18) have been dating for 5 months. I was using her phone yesterday because she took photos of our new puppy and I was sending them to my number. When I was going through her gallery, I saw a photo she took last week. And she was holding up a peace sign with her boyfriend next to her, they were in the car, you could only see their faces so no nudity, and she had sperm on her face. I was obviously livid when I saw the photo and since she fell asleep when I was about done with her phone, I didn’t wake her up and just slammed it down on her desk. She worked a 10 hour shift so I would have felt bad waking her up. I couldn’t confront her today either because I woke up too late to catch her before she could go to work but she comes home at 10 tonight. Im honestly not sure how the conversation is going to go. Most likely I’ll end up taking her phone and restricting her. I also want to have a couple of words with her boyfriend and tell his parents. I’m just extremely angry and surprised right now. This was a surprise for me. I called my sister because I was so angry and shocked and she told me to just recommend her not take photos like that as they can be easily leaked and then leave it be. How am I just going to *leave it be?* What do I do? Or what should I do?