You’ll need kpartx:
sudo apt-get install kpartx
I’ll be using 2012-10-28-wheezy-raspbian.img as an example
List image partitions
sudo kpartx -l 2012-10-28-wheezy-raspbian.img
Add partition mappings
sudo kpartx -a -v 2012-10-28-wheezy-raspbian.img
Mappings are created under /dev/mapper; the -v option additionally prints the list
Mount partition (read-only)
Assuming you don’t have anything under /mnt (make a new mount point and adjust the path accordingly if you do)
sudo mount /dev/mapper/loop0p2 /mnt -o ro
(using the mapping - /dev/mapper/loop0p2 - that was created (and printed) in the previous step)
Unmount and remove mappings
sudo umount /mnt
sudo kpartx -d 2012-10-28-wheezy-raspbian.img