.img files are not images of a partition, but of a whole disk. That means they start with a bootloader and a partition table. First, you have to find out the offset of the partition and mount it with the offset option of mount.
fdisk -l /path/to/image
Fdisk shows the block-size and the start-block of the partition. Use them to calculate the offset. For instance, I have an image of a bootable stick with a 4GB FAT32 partition. The output of the fdisk command is:
Disk Stick.img: 3984 MB, 3984588800 bytes 249 heads, 6 sectors/track, 5209 cylinders, total 7782400 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 Disk identifier: 0x0004bfaa Device Boot Start End Blocks Id System Stick.img1 * 128 8015999 4007936 b W95 FAT32
So I have a block-size of 512 bytes and the start-block is 128. The offset is 512 * 128 = 65536. So the mount command would be
mount -o loop,offset=65536 Stick.img /mnt/tmp