% sudo mv /System/Library/CoreServices/[Some extra line breaks were added to narrow the width of the display; enter each command as one line with no spaces except put a space in the first command before "/Users" and in the second command before "/System".]
loginwindow.app/Contents/Resources/AboutBox.tiff
/Users/your_user_name/Pictures
% sudo mv /Users/your_user_name/Desktop/AboutBox.tiff
/System/Library/CoreServices/loginwindow.app/
Contents/Resources/AboutBox.tiff
You will need to restart to see the changes.
[Editor's note: You should be comfortable with the Terminal before trying the shortcut mentioned above; Andy's tutorial is 100% Finder-based, so it's much simpler if you're new to this sort of stuff.]

