Stolen art on fellow site?

I’m not quite sure I have this in the correct category for support, so any corrections in switching the placement of the thread would be nice.

I have currently been informed that my art from another site, is currently being used on a wordpress powered site, without my permission. Not only this, but art I have commissioned as well, and again, is without permission.

My question is, how would I go about getting it taken down? I certainly wouldn’t have minded it being posted, but do not appreciate the blatant posting not only without a link back to the source, but without even so much as contacting me in the first place.

Any help at all is very much appreciated.