Get informations about partitions in image:

fdisk -lu Udoo.img

Output may looked like this:

Disk Udoo.img: 7,4 GiB, 7948206080 bytes, 15523840 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
Disklabel type: dos
Disk identifier: 0xbf31fe90

Device     Boot Start      End  Sectors  Size Id Type
Udoo.img1        8192 15523839 15515648  7,4G 83 Linux

On my Udoo.img I have one partition, when I want mount this partition need to know position in bytes, where start the partition in image file. My partition start on 8192 block, default block size is 512B then start position is 8192 * 512B = 4194304B

Now we can mount partition:

mount -o ro,loop,offset=4194304 Udoo.img /mnt/temp