how to change the login screen on 10.5 and later

to change the apple logo:

cd /System/Library/CoreServices/SecurityAgentPlugins/loginwindow.bundle/Contents/Resources
sudo cp applelogo.tif applelogo.tif.original
sudo cp mylogo.tif applelogo.tif

to change the login wallpaper image:

cd /System/Library/CoreServices
sudo cp DefaultDesktop.jpg DefaultDesktop.jpg.original
sudo cp mydesktop.jpg DefaultDesktop.jpg

reboot and enjoy!