The more I think on it the more I don't see how it would work, take msn backgrounds for starters, you can't use .eps or any other vector file to let you scale the image smoothly, so it has to be, a what usually ends up as a distorted bitmap, and there is no size requirements, no file too big, no file too small. What would be the official size you'd be required to upload them as? And how many people use the same size window anyway? It'll just end up like a new wallpapers section and without heavy moderation will more than likely end up with people simply resubmitting their walls in smaller format, and a ton of people getting those "groovy" pictures they ripped off the web and spamming them on here too. Emoticons and display pictures I could do without as well, unless you like the idea of ending up like deviant art getting 30 submissions a day of a johnny depp gif that alternates between 2 pictures with text saying "but why is the rum gone?, but why is the rum gone?, but why is the rum gone?"
I just don't see this one working out.