Ruphy set up a page for accepting uploads of wallpaper ideas for Oxygen (thanks Ruphy!). Go to the Oxygen Wallpaper Contest page and upload your pics while they're hot!
If you have a good idea for a wallpaper to use in KDE4 get them in now. Most important is that they look nice - no limits on colors or, well, anything :-) Of course, all pics submited need to be under a free license.
