Just a by-the-way: If you're concerned that one of your images has escaped custody and is running amok out there on the 'net, try Tin Eye. It's a reverse image search. You upload or link to your image, and it searches its index to find matches. It's still in beta, I think, and it's still building its index, but it does work. I've never found one of my own images with it, but I have used it to track down the source of an image that I wanted to use in order to find out its pedigree (it turned out to be a NASA photograph in the public domain, which is why I found it everywhere with no attribution).

And welcome Crayons! The issue of derivative works is murky. I've done essentially what you describe with the map from Robert Jordan's The Wheel of Time, and I have to admit that it made me a bit nervous, particularly since I was unable to locate any guidelines for fan art based on Jordan's work. RPGs, of course, are a somewhat unique area of intellectual property, since there is an expectation that GMs and players will expand upon the existing information and create derivative works as a side effect of playing the game. And since we roleplayers are naturally cooperative, sharing people, anyone who publishes in this industry should expect that those derivative works will be shared and further expanded. That being the case, I do not think Mongoose should object to your project or your sharing of it. Whether should equates to would is, of course, another matter.

The short answer is that there isn't a short answer—maybe it's okay and maybe it isn't, and the way the courts would hypothetically decide shouldn't be your guiding principle. The safest course is to try to get in touch with Mongoose and ask for permission, 'cause that's bulletproof.