The problem with Gimp is that, like Photoshop, it is a full photo editing program. If all you want to do is view photos, it is way overkill.

Debian has some other programs for converting RAW to formats that can be viewed by picture viewers, such as dcraw, ufraw, and rawstudio.

The best thing to do is to search the Debian repositories; Pretty much anything you can find in Debian, you can run on the tablet via Easy Debian.

If you need to download photos from the camera to the tablet, and it doesn't automatically mount the camera like a USB drive, then you will need gphoto2, and that can be tricky to use on the tablet; it is known for crashing the tablet unless used exactly correctly (look near the end of that thread for some tips on using it without crashing the tablet).