It bugged me for a long time that the user pictures in the login window would not respect a user picture's transparency mask, but draw the icon on a white square instead. Luckily I found out how to get rid of this in Mac OS X 10.5! As this solution requires the use of Terminal, the usual "use at your own risk" applies.
Leopard stores the cache where it keeps the user pictures accessible via the command line utility dscl. Let's assume the user account whose picture we want to change is called steve. (Obviously, replace steve with the short name of your account in the following commands). Type the following into the terminal:
dscl . -read /Users/steve Picture
This should give output like this, reflecting your currently-selected image:
Picture:
/Library/User Pictures/Animals/Butterfly.tif
There is also an entry in dscl called JPEGPhoto that contains a hex-encoded JPEG version of this picture. If you delete this JPEG version, the OS will use the TIFF, which may actually have an alpha mask!
You can do so by typing:
sudo dscl . -delete /Users/steve JPEGPhoto
Check the Picture entry again with the first command above, as it might have been reset. Now change this entry to point to your new picture; I assume you have created a new directory inside /Library/User Pictures for your own 48x48 TIFF images. Assuming you'd like to use kermit.tiff from a directory named muppets in the User Pictures directory, you'd use this command:
sudo dscl . -change /Users/steve Picture '/Library/User Pictures/Animals/Butterfly.tif' '/Library/User Pictures/muppets/kermit.tiff'Mac OS X Hints
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