on trademarks I think it kinda depends on if the holder complains... like using windows/xp/vista/etc logos on stuff
A lot is stuff is called xp this vista that
I think if you create your own graphic for it, it's closer to fan art
I don't think fan art should include cut and paste or 'modified' graphics from another source without permission from that source
Jafo had a good explanation of it a while back (more than once, but I think it was from last year I'm remembering) but I can't find it