monline

Approximately 7 out of 10 articles will tell you to overwrite the image file that the OS uses as the login screen background by default. Of course this is not the correct way to go since this image is in the /System subtree (/System/Library/CoreServices/DefaultDesktop.jpg to be precise) and theoretically a system update could write back the original image. Instead set the proper system preference to point to the image of your choice:

To revert back to the default behaviour you've to delete this preference since it was not there originally at all:

The fun thing is that there's a utility called Desktop 2 Login downloadable from Apple's website (but not made by Apple itself), however this little app does the hack the wrong way, by overwriting the default image file.

PS: I think that the preference is working for Tiger too, but the default background image location might be different.