If you set a value greater 1 for screen scaling in system settings -> screen -> scale screen (because using a HiDPI screen) to make the UI look good, the actual image displaying in the viewport area of gwenview gets scaled to. This looks pretty bad, because it pixelates every image. 
If you open a screenshot of your desktop for example, and you view the image in original size one would expect, that everything on the screenshot has the same size than the original component. But if you set a scaling factor of 2, the plasma panel is twice as large as the original one. On top of that everything is pixelated.

Reproducible: Always

Steps to Reproduce:
1. Set display scaling factor greater than one in system settings
2. open any picture in gwenview (best seen when opening a screenshot)

Actual Results:  
The scaling factor is applied to the image, making everything look pixelated and to large when viewing in original size

Expected Results:  
Scaling is only applied to the UI, not the viewport, so that one pixel of the image matches one pixel of the screen

I'm using Lenovo Yoga 2 pro with a native screen resolution of 3200x1800px