For what it's worth, I have now tested gimp, and i find it renders the image full screen *better* than gwenview. (It did take a couple of minutes to turn off *everything* in gimp to get the right view though.)
Considering the G in GTK stands for "Gimp", this might not be a GTK issue after all, although it's quite likely Gimp renders images radically different than anything else out there. a. -- It is the greatest of all mistakes to do nothing because you can only do little. Do what you can. - Sydney Smith