what about the image makes it bootable?
An operating system "kernel" sitting at track 0 sector 0.
EDIT: if "data", or a program that doesn't have the necessary code to finish booting the system, is at track 0 sector 0, then the BIOS' bootstrap code will throw an error saying that an operating system is not found.
EDIT 2: if it's a fresh or unformatted disk, the BIOS is still going to try and look where it thinks track 0 sector 0 is, not find anything and then throw an error.