Q - Do I own any copyrights on the graphics I make?
A - Not unless they are 100% your work. If they contain photographs/artwork/etc by other people, the graphics are not copyrighted to you, and you do not have any rights if the graphics are used by others without credit. And yes, this does mean that LJ Abuse is not going to help you out in any way. Period.
Q - If my graphics do qualify to be copyrighted to me (see above), do I have to register them somewhere?
A - No. They will be automatically copyrighted to you. For example, my current icon is copyrighted to me, because I took the photograph myself, but I did not have to register it with anyone.
Q - Are my graphics copyrighted to me because I posted them on DeviantART?
A - Again, not unless they are 100% your work. About the only exception to this is if you are using stock photos, with permission/credit if the photographer requires it. If your graphic does not follow these guidelines, more than likely you're actually breaking DeviantART's TOS and your deviation can be deleted because of it. I've seen people's entire galleries completely deleted because of this.
Q - But the community rules say credit is a must, right?
A - Yes, we do require that people credit unless the person who made the graphic specifically states that credit is not necessary.
Q - So what if someone steals from the community or uses my graphics without crediting me?
A - Feel free to politely ask the person to credit you, and we may ask them to credit you as well. If they are blatently breaking the rules, such as taking graphics out of the comm and posting them in stolen graphics comms or they are crediting the graphics to themselves, or they refuse to credit you after they are asked to, we will ban them and possibly warn other communities, if applicable.
Q - If it's on the internet, that means it's public domain!
A - This is one of the biggest myths out there, and you're going to get yourself in a lot of trouble if you believe it.
Further information: http://www.whatiscopyright.org