The Raspberry Pi images are whole disk images, and they contain generally two partitions inside. To mount one of those partitions you can use this method.
First, inspect the image file to see the start sector of the partition we want to mount:
# fdisk -l ./Fedora-Server-armhfp-22-1-sda.raw
Disk ./Fedora-Server-armhfp-22-1-sda.raw: 4 GiB, 4219469824 bytes, 8241152 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: 0xeca43266
Device Boot Start End Sectors Size Id Type
./Fedora-Server-armhfp-22-1-sda.raw1 2048 1001471 999424 488M 83 Linux
./Fedora-Server-armhfp-22-1-sda.raw2 1001472 2001953 1000482 488,5M 83 Linux
./Fedora-Server-armhfp-22-1-sda.raw3 2001954 7861328 5859375 2,8G 83 Linux
To mount the third partition, we must get the partition offset multiplying the sector size (512 bytes) by the start sector (2001954).
512 x 2001954 = 1025000448
Then, we create the loop device with the obtained offset and mount it:
# losetup -o 1025000448 /dev/loop0 ./Fedora-Server-armhfp-22-1-sda.raw
# mount /dev/loop0 /mnt/img