...and what desktop---eg KDE, Gnome, XFCE?
And what do you mean by "bootup"?
There are at least 3 stages where you can get some kind of custom graphics:
1. GRUB bootloader--defined in /boot/grub/menu.lst
2. Login manager (eg GDM, KDM)--I don't know how to configure this
3. The actual background image in your desktop--this is settable in your menus