.IMG isn't any kind of standard, so it's hard to tell if it would be supported or not. But my guess is it probably is. One thing isn't clear to me yet: are you specifically booting a floppy (PR#5) vs. just letting the system boot from the Zip disk - which I assume is in slot 7 - which probably doesn't have PRODOS.SYSTEM copied to it yet?
It's just an unlucky turn of events that disks without a PRODOS.SYSTEM on them are the ones you can't create. That's all you really need in order to make a disk bootable. (Of course, you also have to know exactly which disk/slot is actually being booted, as noted above...)