(Click here for larger image)
In an earlier item, links were posted to Jordan Miller's site on altering the images used by the LoginWindow app. If you want, you can carry this bit of fun to more of an extreme.
First, you will have to register as a developer and download the OS X developer tools (instructions are elsewhere on this site). Don't be afraid, you can do a bit of interesting stuff with Interface Builder if you are careful. You can change the size of the image, the colors/styles of the fonts, placement of the buttons, etc. You can see an example of this in the screenshot above. The original screen is at 1280x1024. The image is of the wonderful David Hockney work, Kyoto. It calms me down before I login.
If you'd like to know how to edit the login window extensively, read the rest of this article!
[P.S. -- Are there any Cocoa programmers out there that can tell me what loginwindow is doing that makes the purple background not be captured when I try to capture the screen content? Is it drawing the background directly to video memory? Is there a way to capture the whole shebang?]
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.