I am not a lawyer, nor have any variety of that background. My personal opinion on this matter is, it really depends on two things:
The agreement when you made the art - was it specific to the site or could you re-sellf?
Were you paid for the art?
If you were paid for the art, although you own some variety of ownership, it cannot be resold unless the other party also agrees with it being resold no matter how much you want to.
~judda