The image files for the login page are located at wp-admin/images/ of your installation, and named login-bkg-bottom.gif, and login-bkg-top.gif
Copy the files from the server, and edit them in photoshop or whatever image editing software you like. Then reload them to the server using the original file names stated above. Keeping the names the same means there is no need to hack the wp-login.php file.
This definitely works for mu, probably works for the stand-alone version as well.