Lightweight and efficient app for spotting image differences

I am using Ubuntu 17.10.
I am trying to open camera RAW images in GIMP following the advice I have found in this post about resetting the default file association (in this case I didn't have any other program).
I'm doing it as root. When I try and open the first image in GIMP, this is what happens:
root@rasdhani-gaming:/media/rasd 6add127376 kylbald
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