The easiest way to install loginscreens is:

 1) su to root
 2) dwrite loginwindow ImageFile <pathname/filename>
    Some people keep screens in one directory (such as /LocalLibrary/logins)
 3) logout to login window. 
 4) for the user name enter: exit
 4a)press return for the password
    This kills the window server and forces it to restart. It can take as 
    long as five minutes on an '030 cube.

If for some reason it hangs, and doesn't reload the login window, then you'll
have to reboot.