READ NEXT POST -- very important...
I ran into the same problem... freaked me out for a while... because I couldn't get back into OSX...
So I had to boot into OS9 from a CD.. and I replaced the orginal .tiff file that I had put in its place... ( I had made a backup of the orginal, so I just put it back...)
bighairydog is correct, they changed the login setup so you can't just change the .tiff for the whole panel... all the .tiff is the little blue apple... so you can't change the login panel as far as I know, just the login background...
/System/Library/CoreServices/loginwindow.app/Resources/loginpanel.tiff
This file needs to be the orginal one for the system..
incase you blew away the .tiff, it is a big blue apple that is 63x77 pixels at 72 dpi.. so make a new image that size and replace the file with OS 9
Hopefully that is the only file that Metamorph X changed...
I had just tried to replace the .tiff, and ran into the same issue...
But I put it back and that fixed my problem...
but it is possible that a lot of other files got axed, so you need check the files off your ORIGINAL OSX.1 CD and compare them to see which files you need to replace in this directory...
/System/Library/CoreServices/loginwindow.app/Resources
Boot up with os 9 and fix the files..
Just get the originals off of your OS X.1 CD... --
Hope that helps...