Determine the file system by using the file-utility.
file image.img
Use mdconfig to link the IMG image to a virtual device.
mdconfig -a -t vnode -f /path/to/image.img -u 0
The image will now be connected to the virtual device /dev/md0.
Now mount the virtual device. This example mounts an ext2 image:
mount -t ext2fs /dev/md0 /mnt
You can now see the contents of the image in /mnt.
To unmount the image:
umount /mnt
mdconfig -d -u 0
ISO images can be mounted using this way as well, using the file system cd9660 or UDF and mounting it in /cdrom instead of /mnt.