DSM 3.1 and 1511+, should work on other models.
First you will need to load the loop device kernel module
Then you will need to initialise the loop back device with your image.
Check for a free loop device
If you don't have one do
mknod /dev/loop0 b 7 0
Then mount the loop device to the image.
losetup /dev/loop0 <img>
Then mount the loopback device to the filesystem
mount -t <filesystem> /dev/loop0 <mount point>