In the past, when I had to manage my images (pictures) I used GQview (which started back in 2008). But the application doesn't get many updates, and if an application does not get many updates, it either means it is no longer maintained or that it does its job perfectly. Sadly, for GQview, it is the unmaintained reason (even though the application seems to work pretty well for most tasks). Enter Geeqie, a fork of GQview to keep evolution on the application up to speed.
The Geeqie image viewer is a simple viewer that allows to easily manipulate images (like rotation). I launch it the moment I insert my camera's SD card into my laptop for image processing. It quickly shows the thumbnails of all images and I start processing them to see which ones are eligible for manipulations later on (or are just perfect - not that that occurs frequently) and which can be deleted immediately. You can also quickly set Exif information (to annotate the image further) and view some basic aspects of the picture (such as histogram information).
Two features however are what is keeping me with this image viewer: finding duplicates, and side-by-side comparison.
With the duplicate feature, geekie can compare images by name, size, date, dimensions, checksum, path and - most interestingly, similarity. If you start working on images, you often create intermediate snapshots or tryouts. Or, when you start taking pictures, you take several ones in a short time-frame. With the "find duplicate" feature, you can search through the images to find all images that had the same base (or are taking quickly after each other) and see them all simultaneously. That allows you to remove those you don't need anymore and keep the good ones. I also use this feature often when people come with their external hard drive filled with images - none of them having any exif information anymore and not in any way structured - and ask to see if there are any duplicates on it. A simple checksum might reveal the obvious ones, but the similarity search of geeqie goes much, much further.
The side-by-side comparison creates a split view of the application, in which each pane has another image. This feature I use when I have two pictures that are taken closely after another (so very, very similar in nature) and I need to see which one is better. With the side-by-side comparison, I can look at artifacts in the image or the consequences of the different aperture, ISO and shutter speed.
And the moment I start working on images, Gimp and Darktable are just a single click away.