You will need to be root to replace the nib file. The path for the English nib file is /System/Library/CoreServices/loginwindow.app/Resources/English.lproj. There are other lproj for other locales.
1) Either in the terminal, or using control-click to get the context menu for "Show package contents", find the nib file, and make 2 copies to another location. One is your original backup; double-click the other to open it in Interface Builder.
2) Be careful to not rename or delete the image field, the buttons, or the textfields. Also, do not mess with their connections or outlets. You may have to look at some of Apple's tutorials to understand this. But basically, you don't want to mess with things that the app needs to have the objects interact. (If you screw up, the original is still in place. If you have replaced it, that is why the backup original was made. If it is so bad that login no longer works, you may need to go to OS9 and replace with the original.)
3) You can resize the window, the image, move buttons and fields to different locations, change text. So if you want a different size and shape of picture, you got it.
4) Save, and exit IB. Then replace the nib in the app with your edited version.
This all works, because Cocoa objects are "freeze-dried" into the nib file. As long as you don't mess with characteristics that tie objects and their code together, you can modify the look of an app. (You can sometimes do more by adding actual compiled code, but that is a more advanced topic. NOTE: Please see the discussion comments for an interesting exchange on how malicious NIB files may or may not be -- the conclusion is that they are safe in principle, and you could accept a login window NIB file from someone else without great concern.
I have passed this image/info along to Jordan Miller, but I don't know if he will post it on his site.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010206040016713