You're not confusing the grub splash screen with the desktop background image, are you?
I don't think so. The location mentioned by Jimmy has a 24bpp splash image for grub-pc called moreblue-orbit-grub.png <http://blogs.koolwal.net/2008/12/16/how-to-grub2-and-grub-pc-installing-splash-images/> this is a howto which explains how to use it, but that's basically as simple as "aptitude install grub2" (in case of upgrading like I have, otherwise has grub2 become the default for new installations ? I don't know) baldy.