I was looking for a replacement app for Google's photo viewer and got to this viewer via F-Droid. I have couple of remarks, one of which is the following:

On my phone, when viewing horizontal pictures 'full screen', the quality is ok. Yet when viewing vertical images one can clearly see that images are compressed. Only when you zoom in an image, it will display full quality.
I understand this has to do with speed, but I would expect that if an image is open for a second or so, it will load the full-quality display, both for vertical & horizontal images.