I was trying to mount a partition within an image like this:
# mount -o loop,ro,offset=XXX
whereby I had to calculate XXX with values obtained from "fdisk -l <image-file>"

Turns out there's a MUCH easier method:
# losetup -f -P <image-file>

which will give you /dev/loop0p1 and /dev/loop0p2 (in my case there were 2 partitions in it), which you can then mount as follows:
# mount /dev/loop0p1 /media/rasp1/
# mount /dev/loop0p2 /media/rasp2/

😎

@FreePietje: I always used #kpartx for that, but I see that #losetup is actually part of Debian's mount package, which is always installed.
@xtaran
I *think* that you need to use kpartx (or at least not losetup) when dealing with images using GPT (aot MBR)