Currently, the selection for image to be used for image-based provisioning
is placed at the compute profile. I don't think this is the right place
for this pre-selection.

I can image a world where the default image for operating system would be assigned
in operating systems, and it could be overriden in a hostgroup.

I realize, that there can be multiple images for the same OS
but for different providers. IMO it should be possible to assign
an image to OS/hostgroup per provider and the final selection of image would
be determined by currently selected provider.