Sick and tired of the same old bluish background of the Mac OS X login screen? I have now found out a way to change it out. Simply modify/replace the following files:

/System/Library/CoreServices/DefaultDesktop
.jpg (Leopard users)

/Library/Preferences/com.apple.loginwindow (Tiger users: modify this file)

Once this has been done, open a Terminal window and execute this line at the command prompt:

sudo defaults write
/Library/Preferences/com.apple.loginwindow
DesktopPicture '/Path/To/Picture.jpg' (all on one line)

Once this has been done, log out of your account. When you log out, you should instantly see the changes that you have made.