OS X.5 / X.6 - Change Logon-Background:
Locate your favourite wallpaper (jpg), open up terminal and login as root or type everything with a trailing su:
1. cd /System/Library/CoreServices/
2. mv ./DefaultDesktop.jpg ./DefaultDesktop.bak.jpg
3. cp <pathAndNameOfNewWallpaper> ./DefaultDesktop.jpg
2. mv ./DefaultDesktop.jpg ./DefaultDesktop.bak.jpg
3. cp <pathAndNameOfNewWallpaper> ./DefaultDesktop.jpg
You're done.
Hint: Systemwallpapers are located in /Library/Desktop\ Pictures/

