Hi Steve,
Replacing the background picture should not cause any problems. The image is aligned to the top/left corner and it is not stretched. This is defined in a CSS file that is also part of the the Personas package. Both should survive the application of a service pack as long as no new image/css is delivered.
I am not fully sure about how to change the login screen, but I thought you can configure this in SICF like you could do it for any other service.
Cheers,
Tobias.