Just had a fight to mount an image file (img) that was generated probably with a dd command…
The problem is, because it is a plain copy of the whole HD, including bootloader, etc, the usual mounting apps (tried acetoneiso, and others) simply won't work.
Searching a bit for the answer:
(1) Find the linux partition sector of the image:
$ fdisk -l file.img
(2) Mount from that sector
$ mount -o loop,offset=1048576 -t ext3 file.img /mnt/img
where the offset is n*512 (in my case it was the first partition, or 2048, so 2048*512 = 1048576). And you might need to create the directory /mnt/img (or anything else you want).