Alerted by John Howard's facebook post, I'm spotting quite a few GP artists work on art.com and allposters.com (or .co.uk depending on where you are). It's the same site just different interfaces.
Get on there and check for your work and email them to get it taken down, or take them to court or whatever.
Super lame but this happens fairly regularly these days, sadly.
Clay - it looks like they have scanned the large posters from the GeePee books, so just a heads up in case that's an angle worth pursuing.