To get ready for this change, you need two things: an image you’d like to use, sized to your default screen resolution, and an open Terminal window. Once you have the picture you’d like to use, there’s a secret as to where to store the image: save it in your user’s Public folder, at the top level. The other alternative is to put it in the top-level Library -> Desktop Pictures folder, but that sort of defeats the purpose of not modifying the system-owned folders, doesn’t it? The reason your picture needs to go in your Public folder is that it needs to be available to all users. (There are workarounds, but they require mucking about with permissions. And why bother if there’s no need?)
Once the image is ready and saved in the right spot, switch to the Terminal window, and enter this command:

sudo defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture "/Users/[user's_shortname]/Public/[myloginbg].jpg"

[Copy and Paste the text above] into the open Terminal window. However, before you press Return, you’ll need to edit it a bit for your particular setup. Move back with the Left Arrow key until you’re at the / after [user's_shortname]. Now press Delete to erase that bit, and enter your short username instead. Using the Right Arrow, move to the end of the filename, and replace it with the name of your file—make sure you leave the last quotation mark in place. For instance, here’s what that portion of the command looks like on my system:


"/Users/robg/Public/LeopardX.jpg"

Once you’re sure everything looks right, press Return and provide your admin password when asked. If you typed everything right, Terminal won’t echo anything back after you enter your password—you’ll just get the command prompt back. To test your new background, the easiest thing to do is enable Fast User Switching (in System Preferences -> Accounts, then click the Login Options item below the account list). In the menu bar, click the Fast User Switching entry and select Login Window. If the command worked, you’ll see your new login screen. If it didn’t, you’ll probably see a solid blue background instead. If it didn’t work, try it again and make sure you have the command correct, with the proper filename (and filenames in Terminal are case sensitive).


If you ever tire of your custom screen, you can restore the default version by deleting the entry you created in the preferences file:


sudo defaults delete /Library/Preferences/com.apple.loginwindow DesktopPicture

The next time you see the login screen, it should be the familiar Aqua Blue (10.4) or deep space picture (10.5). Although this method is perhaps a bit more complicated than others, it’s a very safe way to replace the login window image without fear of damaging your system.