Sarte, Truthfully, that's the sort of thing you should always do in any case. However, that doesn't, in and of itself, meet the criteria for legality, which appears to me to be the basis of the decisions on the part of Renderosity. Note: the overwhelming majority of license holders don't really have a problem with fan art, insofar as it is. In most cases, the only reason they really go after people is what they consider inappropriate use, or when someone challenges the mark that protects that property and they need to demonstrate good faith protection.

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)