As I found out in some forums (i.e. this ancient Mac OS X Hint), the common way to change the login window's background picture is to replace the Aqua Blue.jpg in /Library » Desktop Pictures with your own image. However, there's a more elegant way to do this: by adding a value to the appropriate plist file, namely /Library » Preferences » com.apple.loginwindow.plist. You can do this in two ways, either with a Terminal command or by directly editing the plist file using Property List Editor from the Xcode package.
Terminal solution:
Open Terminal and type these two commands (the $ is the command prompt; don't type that):
$ sudo -i
$ defaults write /Library/Preferences/com.apple.loginwindow \
DesktopPicture '/path/to/replacement/image/file.jpg'
Replace the /path/to... bit with the full path and filename of the image you'd like to use.
/Library/Desktop Pictures/Plants/Agave.jpg
...to use the Apple-supplied agave plant picture. Please keep in mind that you need the correct user permissions to edit the plist file; save the changes and quit the editor when done. After using either method, log out and you should see your new login window background picture.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070610164328933