You can make a post as a guest to most threads, or start a new thread, as a guest, and report the theft that way.
WHAT WE NEED:
A direct link to the original piece{s}.
A direct link to the place on our site where the image{s} is being used.
That's it! Your post will go into the private moderation queue, but all moderators will see it, which is what matters. We respond to an art takedown notice with removal of the image, usually within 24 hours. Feel free to report it again if it goes longer than that--that would be unusual, but just in case.
We WILL remove it on you the owner's request. Plagiarised works of writing can also be reported in this way.
I hope this helps, and our apologies in advance to anyone who has had their artwork used here when they didn't want it to be.