Mount ISO files

After some additonal testing I found the solution myself: kpartx

from the directory the image files resides in, open a terminal and type:

sudo kpartx -a imagefile.img

Where imagefile.img is the name of the file you want to mount.

If kpartx is not installed, it can be installed using:

sudo apt install kpartx

This command created /dev/mapper/loop0p1 (and for second partition if any) /dev/mapper/loop0p2. Afterwards these partitions can be mounted straight forward using nemo of nautilus.