The fact that they have put the artists name in the bottom corner leaves you open for copyright infringement by posting the image here without their permission, ha, ha!
Basically if you had signed and dated the original art (eg:- B.Bloggs 2001) your attorney could now be demanding a nice fee for its unlicenced useage. I did a scrollsaw pattern as a freebie after 911 and gave full permission to all to post and pass it on, but one little scumbag deleted my name and passed it off at his website as his own creation, I threatened litigation and he closed his whole website next day. My advice is dont deface your work but do sign it, by the way if you want a neat copyright symbol to put in front of your name hit number lock on your keyboard then press Alt and type 0169 ,after letting go the ALT key you get (eg:- Bill Bloggs 2002.)
I do sympathise and fully understand your anger at having your artwork ripped off.
Regards, Tommy.