Hey, I'm going to show you how to change the background picture on the login screen in ANY version of Mac OS X. This Instructable may be used or rewritten in ANY way possible! The log in screen picture below was from Google. I just used it for clearer reference.
Step 1: Navigating to the Correct Folder
Open Mac OS X Finder (new window) and Navigate to: your hard drive with Mac OS X installed, then go to Library and then in Library, go to DesktopPictures.
Step 2: RENAMING!
In my case, I run Mac OS X version 10.4.11 (The Very Latest Version Of Mac OS X Tiger). So everybody that runs Mac OS X Tiger knows that for the login screen, the Aqua Blue Swirl Thingy is the default image. So, to change this, find the file labeled "Aqua Blue" and rename this file to "Aqua Blue2" or if you have more that one Aqua Blue image, name them all 1,2,3,4 and so on. Open up a new finder window and find the file (Picture) you would like to be the login screen's background when you boot up your Mac. When you have found the file, drag it over to the window with DesktopPictures pulled up. Whatever it's name is, rename the file to "Aqua Blue" (WITHOUT THE QUOTES).
Step 3: Is It Working?
When you log out or the next time you reboot Mac OS X, it should show the image you selected!!