Angus is right, those are those little badges put over the disks by the firmware boot selector (option key at startup). Technically this is BootX running, which doesn't handle the drawing of the background.

According to O'Reilly's Mac OSX for Unix Geeks this is the boot order for OSX:

1- Firmware initializes hardware
2- BootX draws the Apple logo and spinning disk. Loads the cached kernel extensions and any new extensions. Starts Mach kernel.
3- Mach kernel initializes BSD subsystem and I/O Kit, mounts root filesystem. Launches init process.
4- init process runs rc script, decides which system services should be started by reading hostconfig. If the system is booting from a CD, init launches the installation program. If you're starting in single-user mode init will drop you into a shell. Otherwise init mounts local filesystems, loads kernel extensions and launches...
5- SystemStarter draws the boot screen and progress bar (and blue background) and loads the StartupItems, then starts the login program

So basically you should look in SystemStarter to change the background, not BootX, which is many steps beforehand.