Neat for when you get tired of looking at the same ol' image.
While attempting to get rid of the gaudy pink and purple Aurora login picture, I found that it is no longer enough to simply rename your picture of choice to Aurora.jpg and drop it in Desktop Pictures a la Aqua.jpg in previous releases.
To change the picture behind the login panel in 10.5, one must name the picture of choice DefaultDesktop.jpg, and drop it in to /System » Library » CoreServices. Doing this as the admin user is required for authentication purposes.
[robg adds: It's the simple things that can make using one's OS of choice more pleasant :). I used Terminal to create a backup of the original image first, just in case. After cding to the CoreServices directory, I then did sudo mv DefaultDesktop.jpg DefaultDesktop_old.jpg, then moved my new image in.]