What I am looking for is a quick and simple photo viewer, like the default Eye of Gnome, but with the one extra feature of being able to press a key on some of the photos to toggle a "selection" flag, and then at the end being able to do a global action on all the selected photos, such as copy them all to another directory.
Use case: I come back from a trip and I've taken upwards of 6000 gigantic photos with my DSLR, of which I want to select the "best" ones (about 1 in 20 or so). I batch-resize them to a size like 1000x1000, suitable to see reasonable detail about sharpness etc but small enough that I can flick through them quickly with Eye of Gnome. But then, when I find one I like, I have to note the number and go through the file system to copy it manually to another directory by drag and drop, which is a pain. Wouldn't it be much nicer if I could just press space to mark the ones I like, and copy them all later? (In fact what I'd do is take the file names of the selected photos and, from that, go back to the original large ones and process them.)
EOG does have a feature whereby you can control-click on the image gallery to select multiple pictures, but it's essentially useless when you're viewing thousands at a time because as soon as you click on a new picture you lose the previous selection. (Same happens if you control-click in Nautilus while using previews, which by the way are too small to see if a photo is sharp.)
I hope someone can point me at a viewer that is as quick and simple as EOG and already has this feature or something close to it. I am not in the least interested in programs that insist on importing all my photos into their "library" in the style of Apple---I want to keep working with my photos organised just with the plain vanilla file system folders.