Are you not a big fan of the new login wallpaper in Leopard? I wasn't a huge fan either and luckily, there is a pretty easy way to change it, and I'll tell you, and show you, how. Just a disclaimer, I am NOT responsible if you screw up your Mac in any way, shape or form. This process worked for me and it should work for you. Also, CoreServices contains some important system stuff, so if you don't know what you're doing, don't do it.

Read the how-to

or

Watch the screencast