This post will show you how to change osx mountain lions new notification center background image.
1) open finder
2) goto /System/Library/CoreServices
3) right click on notification center.app
3) click on show package contents
4) go into the Contents/Resources folder
5) replace the file linen.tiff with the tiff of your choosing :)
6) quit the “Notification Center app” from the activity monitor (Applications>Utilities)