Check this document for more about the image http://docs.openstack.org/developer/ironic/deploy/install-guide.html (Ironic Install Guide)

Starting with the Kilo release, Bare Metal service supports local boot with partition images, meaning that after the deployment the node’s subsequent reboots won’t happen via PXE or Virtual Media. Instead, it will boot from a local boot loader installed on the disk

Reading the doc a whole disk image is a fully function image ready to laydown on the drive.