Feel free to use stocks, textures, brushes etc.
Then, after no predetermined period of time I will post everyone's pictures and we can do a judgey thing if you want.
Now, I'd rather it if you would post the .psd of your wallpaper than the .jpg, because I'm interested in seeing everyone's photoshopping techniques. But if you don't wanna do that, that's fine too. If you want to know a good place to upload files (such as .psds) then I would suggest mediafire, or perhaps box.net.
I'm thinking of making a community of challenges, both icon and wallpaper, depending on how well this goes.
Oh, and all comments with the finished wallpapers are screened.