well there are actually 3 screens your phone goes through on bootup.
The 1st is where it says AT&T or whatever service you have.
The 2nd is an animation
The 3rd is the Windows Mobile screen, right before its completely loaded.
This picture i posted replaces the 3rd splash. Save the picture as welcomehead.96.png (make sure its a png file, not a jpeg), and place it in the Windows folder on your phone.
The 2nd animation is a gif file in the Windows folder, but i can't remember its name off the top of my head...i think its animate.gif
search around the forums