Tim & Staale have encouraged the community to embellish and redistribute their work for FREE, but I've never heard their policy on modifying and then SELLING their works. IMHO, (and only MHO) That would seem a bit too cheeky. Of course it is up to the original author, but this is a tough issue for which to draw an ethical "line in the sand". For instance, one might do something as insignificant as changing the nail color on one of their maps and then try to sell the whole image as a new work. At the other extreme, most map makers often keep things like the inner mouth, tounge, or teeth textures that came with the original figure, yet the rest of the image may be completely new.