I'm going to come in from a different angle. Sure ofcourse I'd want to know if I were either parent, but I don't think the girl in the picture is a victim. She had to take the pic and send it, the boy could have just been the person who received it. Although it is alarming that this is being done at such a young age, I think had we all grown up in a time where we all had cell phones at 13 & younger many of us would have been experiencing this too. I would hope that the parents of each child are being active in their childrens lives - and even if they are that's not going to stop the teens and their hormones. I'm affraid there is not a whole lot we can do aside from educate our own children about the use of cell phones, internet, facebook, and photo sharing like this can do to ruin lives in the future.
Maybe I'm the only one who read this that thinks the real issue here is not the photo, its the lack of trust between the both of you. I mean this in the best most constructive way possible - If your having major trust issues with your step daughter, I can definitely understand why. She's a teenager and I remember being a teen who pulled back from my parents also. You can't go around snooping in her stuff and reporting to her friends and her new "boyfriend's" parents which would be the equivalent of in the teen world "ruining her life!" It's not a great situation but its not a life/death one either. Kids are doing this ALL THE TIME now - super unfortunate - and scary and I don't even want to think about what its going to be like when my kids are teens :-/ !!! But, If you want her to want to confide in you, you need to show her she can! She needs to be able to tell you things, things that you DONT want to hear and trust that you WONT go telling everyone and can provide her with guidence without condeming her and allow her the room to be able try to make the right choices herself. If she or her friends are having sex or sexual relations you are not going to stop it by telling the parents, that alone will probably make her push further away, give her more reasons to lie and be secretive. Use this as a learning point for the both of you. Sit down with her as equals - not shaking a finger, let her know what you found and that your sorry you felt the need to snoop but your concerned about her and her lack of talking and disclosing important information with you. Teach her why its a bad idea to send photos like that and teach her about the sexting. Look up stories of how social networking/photos such as these have ruined lives (there are plenty! One I can recall recently where a boy lost his football scholarship over a couple tweets: http://www.theblaze.com/stories/catholic-high-school-foot...
Even let her help you do some research, it would probably catch her attention and break the ice with some "dirty" topics she might have questions about if you both are able to both research, read and talk about real people & real stories. Maybe after doing this, you can both come to a conclusion of what to do about the situation, or what she should do if she comes across this type of situation again. Sounds like you both have work to do on both ends of the relationship. Tell her you'll work on not snooping, if she works on being more open with you. You can't lock em up, but you can teach them and guide them and hope and pray that they listen, learn and make good choices and learn from the bad ones they've made! Anyway my comments are just ideas to try to gain trust between you.
Bottom line though, you need to do what you feel is right, I dont know the whole situation, but I do know if you are having trust issues going and telling everyone based on a note you found is going to be a major set back. The most important part in this is your stepdaughter, she's where you can make a difference. Good luck!