# mount -t auto -o ro,loop gmapsupp.img /mnt/iso/
mount: you must specify the filesystem type
# file -k gmapsupp.img
gmapsupp.img: x86 boot sector, code offset 0x0
#

$ file centostest.img
centostest.img: x86 boot sector; partition 1: ID=0x83, active, starthead 1, startsector 63, 208782 sectors; partition 2: ID=0x8e, starthead 0, startsector 208845, 20755980 sectors, code offset 0x48

$ sudo /sbin/fdisk -lu /kvm/centostest.img
last_lba(): I don't know how to handle files with mode 81ed
You must set cylinders.
You can do this from the extra functions menu.

Disk /kvm/centostest.img: 0 MB, 0 bytes
255 heads, 63 sectors/track, 0 cylinders, total 0 sectors
Units = sectors of 1 * 512 = 512 bytes

              Device Boot      Start         End      Blocks   Id  System
/kvm/centostest.img1   *          63      208844      104391   83  Linux
/kvm/centostest.img2          208845    20964824    10377990   8e  Linux LVM
Partition 2 has different physical/logical endings:
     phys=(1023, 254, 63) logical=(1304, 254, 63)

$ mount -o loop,offset=512 centostest.img /mnt/tmp

$ mount -o loop,offset=7168 centostest.img /mnt/tmp