This fails to consider the possibility that the display hardware will be tampered with. It also does not consider if a copy of the picture is made and is then displayed by a separate program that pretends that the booting is slower than it actually is.

> Would need some way to prevent anything else from drawing that section of the screen until you're past that stage of boot.

It might need to prevent drawing anything on the entire screen. Otherwise a program might be able to modify the resolution, refresh rate, etc, to try to hide the picture or to display a different one.