PLEASE LOOK UP YOUR USERNAME ON DERPIBOORU. (and if you want, look up your friends usernames too, so you know they haven't been stolen from as well)
I found over 30 of my pictures there, here's how to look it up/report it:
http:// derpiboo. ru/tags/artist-colon-USERNAMEHERE (NO SPACES)
Check your old username too if you changed usernames, and check your other accounts as well.
It's easy to report, all you have to do is scroll down to report image under the picture, but i think you have to have an account, which too are easy to make
They do credit, but I would be uncomfortable with my artwork posted somewhere else, even with credit