Please note this is for Mac OS X Snow Leopard 10.6
Change the Login Screen Wallpaper Image in Snow Leopard 10.6 (also works for Leopard 10.5)
Launch the terminal and issue the following commands:
* cd /System/Library/CoreServices
* sudo mv DefaultDesktop.jpg DefaultDesktop_org.jpg
* sudo cp /path/of/image.jpg DefaultDesktop.jpg
Of course, change /path/of/image.jpg to the path of the image file you want to use. Basically what you’re doing here is moving to a folder, backing up the old “DefaultDesktop.jpg” by renaming it, and then copying in the new image and naming it “DefaultDesktop.jpg” instead.
Edit: Don’t pay for apps that do this for you, the method above works great and it’s totally free, If you are like me you will change your desktop wallpaper quite frequently and also you can now change the boring login window within OS X : ) enjoy!