defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture /Path/to/better.jpg
thought that would work nice, but so far it only disables the picture, and then works when i set it back to the default.
defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture/System/Library/CoreServices/DefaultDesktop.jpg
eh, more thinking later.
