And please add the photos from the color target?
Instead of libexif2 you could use gexiv2, it's an GObject wrapper around libexif2. Perhaps that's easier in an GTK3 program.
Could you please add a license for the images. For this test images I would prefer CC0: https://de.wikipedia.org/wiki/Creative_Commons#CC0
It would be nice if you could make the UI translatable.
Sadly the link at the end of you blog post https://blog.brixit.nl/pinephone-camera-adventures-part-3/ to the example images does not work. I think it would be best do add them directly to the repository.