haha, Walter :)
You can use the rucklidge algorithm. It relies on the haussdorf distance of a set of points, say the 10 points of your pentagram, and looks for a similar set of distances in all points of the target image. It takes a while sometimes, but it's pretty good.
You can also decompose the image with wavelets and look for similar signatures that way I've heard, but I can't help you build something like that. There may be something in the file exchange, like train a face recognizer to look for pentagrams or satanic faces.