you can customise the default blue login screen, but not the startup screen (or at least you couldn't in 10.1).
there are some threads on this forum somewhere - probably from around september / october of last year, also try looking at: http://www.ResExcellence.com/ they have a section of modifications.
you could try changing the file called "Aqua Blue.jpg" in /Library/Desktop Pictures/ and it should get displayed instead of the default background - obviously this would also change your desktop picture if you had it set to this image.
Here's an app that'll do it for you. There's also an app I've heard of called Visage that'll also let you change the Boot and Login Panels. Never tried it and don't know if it works with 10.1.x