Change the Login Background Wallpaper on your Mac
If you want to change the wallpaper that sits in the background of your Mac’s login screen, from the Finder hit Command+Shift+G to open the ‘Go to Folder’ screen. Type /System/Library/CoreServices/ and click ‘Go':
Next, find the image file called ‘DefaultDesktop.jpg’ – you should save a copy of this somewhere safe, or rename it to DefaultDesktop-backup.jpg or something of the sort, so that you can revert your changes.
After that file is safely backed up, add the new wallpaper image you want to use as the login background, naming it exactly the same as the previous file: DefaultDesktop.jpg
That’s all there is to it! Reboot and your login wallpaper is now whatever you set it to. This works to change the login background in Mac OS X Snow Leopard 10.6