I accidentally see something horrible on facebook...?

that's a virus Don't CLICK just hide the post

How old at you? You should be 13 to sign up to Facebook and it's the internet, you'll come across pictures and things like that from time to time. Your parents should already be aware of this, you could just forget about it and move on. But it may be a good idea to tell your parents, just so they are aware of what you may come across online and on Facebook.