The MacOSX.tif is located with the Resources folder of the loginwindow app.

As always save the original file somewhere else so you don't lose it.

Replace it with a downloaded image or one of your own cool creations.

Navigate to the StandardMenus.nib as shown in the illustration and make a copy to work from on your desktop.

Open the copy using Interface Builder and change the name of the menu item to whatever you want it to be.

Save your new copy to the desktop then switch it out with the original, don't throw away the old one - save it somewhere safe so you've got it for backup.

Log-out or relaunch the Finder to see your change.