It's in the menu.lst (or grub.conf depending on distro). Probably /boot/grub/menu.lst
That graphic is very limited though. Of the top of my head something like 640x480 and 14 colors. (not 14 bit, but 14 actual colors in the palette of the image) http://www.gnu.org/manual/
should have the actual specs and the likes. You probably upgraded your grub version and it overwrote the previous config or something like that. Or that image got moved / renamed / corrupted.
Strange... References to that graphic seem to be missing from the manual now. Perhaps a feature they removed in recent versions of grub.