Virtual machine images typically don't need much security, so the login prompt is more of a formality than anything else. Plus, if you're planning to share the VM image with others, you need to communicate the login information along with the image. It's a pain.
I've seen tips on how to force the login background to be an image containing the username and password which appears directly above the login dialog.
I don't know why you would bother with the "bitmap login background" method, as the autologin method is so much cleaner.