If you typically store your image collection on a disk volume other than /boot, and then try to use one of those images for your Desktop background, you’ll find that the image is no longer your background the next time you boot. This is because the boot volume is mounted first, the Desktop is drawn, and then the other automountable volumes are mounted. To avoid this situation, you might want to create a directory on your boot drive just for storing background images.
If you’re feeling brave, you can mount the image volume sooner in the boot process by using the mountvolume command from within the system Bootscript (/boot/beos/system/Bootscript), rather than from within UserBootscript. But remember — you’re officially not ever supposed to mess with system-level components like Bootscript. Do this only if you know what you’re doing.