I Need Help
Contact the website owners tell them it was mistakenly posted, that you accidentally posted it, and take it down. Most sites only let the original poster request it taken down. If they knowingly have an under age pic on their site, they could get,in more trouble than you.
You definitely can get legal charges against you. I don't know how you can undo it. An acquaintances daughter was a minor and was enticed by an adult to post nude pics of herself. SHE got charged with something to do with publishing pornographic pics of a minor. Pics of HERSELF. She looks gorgeous, but there is clearly something a little off in her mental capacity. Clearly she was taken advantage of and clearly she is the victim. Yet the state of Minnesota prosecuted her.
The person who enticed her remains anonymous and wasn't charged cuz he knew what he was doing and covered his tracks. They don't know who, where, or what he is. The why part is obvious. She is beautiful. And an adult by now. You may need a lawyer. But don't hire one unless you are charged with anything. And don't talk to any cops or gov people if you are questioned. You do not have to talk to anyone. If they are questioning you without arresting you, that means they don't have enough evidence to arrest you. Until you talk to them and reveal your connection to it.
Let people here know how it ended.