The answer by @Catskul and @Cristian Ciupitu is perfectly fine, but it misses the loop unmount command. So if you have to do a second image, you will end up with using loop1, loop2 etc.
you can check which loop devices are connected to which images by calling losetup:
pk:~# partx -v -a /home/pkolmann/img/Test.img
partition: none, disk: /home/pkolmann/img/Test.img, lower: 0, upper: 0
Trying to use '/dev/loop1' for the loop device
/dev/loop1: partition table type 'dos' detected
range recount: max partno=2, lower=0, upper=0
/dev/loop1: partition #1 added
/dev/loop1: partition #2 added
pk:~# losetup
NAME SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE DIO LOG-SEC
/dev/loop1 0 0 0 0 /home/pkolmann/img/Test.img 0 512
/dev/loop0 0 0 0 0 /home/pkolmann/img/Test.img 0 512
after unmounting the partitions with
pk:~# partx -v -d /dev/loop0
partition: none, disk: /dev/loop0, lower: 0, upper: 0
/dev/loop0: partition #1 removed
/dev/loop0: partition #2 removed
pk:~# partx -v -d /dev/loop1
partition: none, disk: /dev/loop1, lower: 0, upper: 0
/dev/loop1: partition #1 removed
/dev/loop1: partition #2 removed
the loop devices are still used:
pk:~# losetup
NAME SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE DIO LOG-SEC
/dev/loop1 0 0 0 0 /home/pkolmann/img/Test.img 0 512
/dev/loop0 0 0 0 0 /home/pkolmann/img/Test.img 0 512
These need to be removed extra:
wspk:~# losetup -d /dev/loop0
wspk:~# losetup -d /dev/loop1
wspk:~# losetup