Rather than posting an image and having people render the character(s) from the background, why not post a pre-rendered character (.png) and let people do whatever they want with it to see who can be the most creative? A banner, signature, wallpaper, etc. I've always wanted to see what different people could do with one common source material.
That actually sounds like a great idea. I think we should go with this except make what we are making with the image more specific (i.e. make a sig with this image or make a banner using this background image)