This is the hypothetical letter I will write if I discover my daughter is posting racey "selfies". The facts can be adjusted should this come up.
I have printed this letter and attached a printed copy of the selfie you took of yourself recently so as to keep this private. No, I didn't hack your facebook account, or sift through your phone, or do anything to openly violate your privacy. This picture came to me via an online acquaintance of mine who happens to be the mother of a friend of yours (small world isn't it). You friend happens to be a boy. If he is anything like the man I hope he will become then he will never touch you, and probably never speak to you again. You can not "unsee" what has been seen. But aside from consequences to your social life, let's talk about real life.
Did you know that posting overtly sexual or naked "selfies" can get you tagged with a sex offender charge? You can be put on the sex offender registry and that's a black mark that can ruin your life for years.
Did you know that future employers may do an online search for information about you and find that "selfie"? That picture can ruin your job prospects. Very few employers want a bad reflection on their company by hiring someone who projects nymphomania.
Did you know that I love you and want better for you in life that what you are asking for with that picture? I can take away your phone, you camera, your bedroom door. I can try to stop you from seeing friends and demand that while you live in my house you live up to my standards. However, ultimately I can't stop you from doing what you will do. One day I will not be there to shield you from the full consequences of your decisions. The best I can do is try to teach your right from wrong. I hope I've done so.
P.S. You're grounded.