* If -p option given, use that project to find the image. Fatal error
  if no such image in that project.

* If no -p option:

     * If in -e (pid,eid) mode, then first look in the same project as
       the experiment (pid), and then look in emulab-ops.

     * If not in -e mode, then look only in the emulab-ops project.

     * If not obvious, -p option always overrides the search.