Mounting an ISO/IMG on Linux / Raspberry PI

To mount an ISO/IMG file in Linux (or your Raspberry PI), first information on the contents of the image must be found using the fdisk command:

pi@pi_horizon_emb:~ $ fdisk  -lu /media/pi/41AB-0764/2016-03-18-raspbian-jessie-lite.img
 
Disk /media/pi/41AB-0764/2016-03-18-raspbian-jessie-lite.img: 1.3 GiB, 1361051648 bytes, 2658304 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x6f92008e
 
Device                                                   Boot  Start     End Sectors  Size Id Type
/media/pi/41AB-0764/2016-03-18-raspbian-jessie-lite.img1        8192  131071  122880   60M  c W95 FAT32 (LBA)
/media/pi/41AB-0764/2016-03-18-raspbian-jessie-lite.img2      131072 2658303 2527232  1.2G 83 Linux 

Note the block size and start location of the partition you wish to mount.
Then mount the partition within the image to a location of your choice on your local disk. Note you need to specify the offset, which is the start location * the block size.

pi@pi_horizon_emb:/ $ sudo mount -t auto -o loop,offset=$((131072*512))  /media/pi/41AB-0764/2016-03-18-raspbian-jessie-lite.img ~/iso_mount/
pi@pi_horizon_emb:/ $ ls ~/iso_mount/
bin  boot  dev  etc  home  lib  lost+found  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var
pi@pi_horizon_emb:/ $

You can then modify the contents of the image partition. Note the partition size is restricted (it won't just expand as you add to it):

pi@pi_horizon_emb:/ $ df /home/pi/iso_mount
Filesystem     1K-blocks   Used Available Use% Mounted on
/dev/loop0       1210960 732164    399232  65% /home/pi/iso_mount