It's preferred that the work be original, but if you want to make Mario squaring off with Sonic or something, that's fine with me. Don't forget, lighting, atmosphere, and mood are the main judging points. And the big goal is for you to end up with a nice portfolio piece.
there is no specific resolution, but we do ask that if it's over 640x480, to please host the image yourself as opposed to attaching it to the forum, for bandwidth reasons.
Minor post tweaking is ok.