To change the login "panel" in X.3 that Visage used to be able to do in X.2, modify CoreServices/SecurityAgent.app/Contents/Resources/applelogo.tif, but you have to ensure that the file is as identical as possible -
Pathfinder (via Psuedo) can do this part -
User: root
Group: wheel
permissions: -rw-r--r-- (644)
NO creator or type
NO data fork
Graphic Converter can do this part -
use same pixel size and color palette
Another location, CoreServices/SecurityAgentPlugins/loginwindow.bundle/Contents/Resources/, has the same resources, but changing them seems to have no effect.