So the base question is in the title.
I know one application which can show big images in full resolution — QuickPic
https://play.google.com/store/apps/details?id=com.alensw.PicFolder — but it's nonfree and owned by Cheetah Mobile which has bad reputation.
As a precautionary measure, I'm seeking for alternatives which can also show big images in full resolution and are (ideally) free as in freedom, or at least are well known to not mistreat users.
All else being equal, the less other extra features it has, the better. But if it's open source, I can trim it down myself, of course.