My favorite image viewer on Linux
…is Gwenview. Had forgotten the name, so it took me a bit of searching to find it and install it again. It has function keys for moving and copying images, which makes sorting easier. It also has of course a slideshow.
I just found one called geeqie that seems to do the same things, but also allows you to tag images into six categories with keypad 1-6, and then do stuff with a tagged set of images..