What did you do? Chances are if anything happened it would just be a warning from your ISP. They don't have the time or resources to go around arresting everyone because of something they did on the internet.
If you were using a proxy the chances are slim to none that anything will happen. There are ways they can track down your real IP address, but unless it was something major they aren't going to bother.
Edit
Was it on stickam? I used to see people do that all the time there just to shock groups of girls, don't worry about it. Unless you showed it to a minor, then you could be in some trouble. Even then though, more than likely nothing will come of it.