You should mount the bigger disk and then mount the loop device to it.
I don’t know which OS you’re using, but it’s some kind of Unix, so this should work:
Connect the external drive.
Check if the disk is mounted. If it is, then copy the path it’s mounted, otherwise mount it. To check if the device is mounted, run lsblk and find the disk that matches (should have a similar capacity).
To mount the external drive (if not mounted), run:
sudo mkdir /mnt/external
sudo mount /dev/sdb1 /mnt/external
This assumes your drive has only one partition and the device is sdb, which depends on how many drives you have.
Create a folder under the external drive: sudo mkdir /mnt/external/rescue
To mount the rescue disk:
sudo losetup /dev/loop0 rescue.img
sudo mount /dev/loop0 /mnt/external/rescue
Now you should have access to the data. The steps may vary depending on Your OS and how it’s configured, but it should give You some hints .