You may have to convert them to ".nb"
Please comment!
Original bootscreens are at
http://forum.ppcgeeks.com/showthread.php?t=16379
Enjoy!
EDIT:
I just realized that the UTstarcom one is slightly unaligned, but it seems that you can't edit attachments, so you will have to fix that one yourself. Sorry!
EDIT AGAIN:
The Bender one is supposed to look like the bootscreen of on old Macintosh.
The Peter Griffin one is supposed to look like the Sprint logo is about to hit him in the face.