Microsoft while announcing Xbox Fun Downloads which has Wallpapers, emoticons and other goodies and it's the downloads for people who register for Xbox LIVE also provided Xbox JumpIn Wallpaper available for all.
More details at:
Here is the Xbox JumpIn wallpaper for all.
Download them from here:
Wallpaper_JumpIn_800x600.jpg size:579 KB
Wallpaper_JumpIn_1024x768.jpg size:978 KB