now, how do i make my own?
Trick question? Create a graphic file with a graphics/image editing
program. Use Paint (very, very basic) or something more elaborate such as
Photoshop.
Having the image you created "fed" to startup by a third party program such
as the one that Shenan suggested would be my method of choice for changing
that screen. There is another method for inserting your image into startup
but it involves altering a core system file so don't recommend it.