Mac OS X: Hacking Mac Login Window

a GUI software called Cocktail is a handy one.

Here is a hacker guid for changing Login window looks:

Change the picture file to your own, size is 90x90 and tif file format are essential.

over write this file: /System/Library/CoreServices/SecurityAgentPlugins/loginwindow.bundle/Contents/Resources/applelogo.tif

Or if you have xCodes installed, you could make copy of loginwindow.bundle and open it with Interface Builder, double click the Apple logo, in the Inspector window, change the applelogo.tif to your own tif picture file and copy your file to /System/Library/CoreServices/SecurityAgentPlugins/loginwindow.bundle/Contents/Resources/, and copy back loginwindow.bundle to /System/Library/CoreServices/SecurityAgentPlugins/, change ownship and privelleges. Youe'r done!