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.

But there's an even easier solution. Tweak UI, one of the official Microsoft PowerToys, allows you to enable a default login, with no typing at all. It calls this feature "autologon":

I don't know why you would bother with the "bitmap login background" method, as the autologin method is so much cleaner.