**** UPDATE FOR OS 10.8 ***
Changing the login screen background requires moving a few image files around and renaming them.
Go to the following folder and find the file named applelinen.png.
Replace this file with whatever background image you want to set, and rename your file appleLinen. You will need administrative rights to make this change, and should use a 1400×900 image for best results, although it will depend on your display size as well. The image should be in PNG format. Once changed, lock your screen and your new background will replace the default one.
Here is how to change the default Mac desktop wallpaper splash screen shown when logging into OS X if space and stars aren’t your cup o’ tea. All you need to do is replace the file DefaultDesktop.jpg located in /System/Library/CoreServices/DefaultDesktop.jpg. Since this is part of the System, you’ll need administrative privileges to change the image.
Open up the Terminal in Applications » Utilities and type in the following:
sudo mv DefaultDesktop.jpg DefaultDesktop_org.jpg
sudo cp /Path/to/picture/you/want/to/use/image.jpg DefaultDesktop.jpg
After the first “sudo” command, you will be asked for your password. For the last line, instead of manually typing in the path of the image you want to use, you can just drag it into the Terminal. If you were to browse to the CoreServices directory in Finder after you did this, you’d see something like this showing the new image and the renamed old one. You will need to change the permissions on the file to allow read from everyone. ("chmod 744 DefaultDesktop.jpg" will do the trick if you are logged in as root)
Alternatively, you may opt to go the “proper” route and add an entry into the appropriate plist file with (below is all one line to be run in Terminal):sudo defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture "/Path/to/picture/you/want/to/use/image.jpg"