This is always a very tough issue for the moderators of WinCustomize because no one enjoys rejecting the hard work of graphics designers and other artists.
But we want to make sure the site stays focused as a skin site first and doesn't end up flooded with wallpaper. So we're starting to become increasingly pickier on what wallpapers end up on the site.
We generally focus on wallpapers that work as wallpapers (as opposed to just being artistically nice). On top of that, unique-ness helps. Using Bryce to create a landscape is very unlikely to make it up on the site for instance.
I just wanted to write this up so that there are no hard feelings on this.