Hello, I wrote some documentation about gThumb, an open source image viewer and organizer for the Linux platform.
This is a very practical program, but the actual documentation is a bit brief and somewhat outdated and hiding therefore some of its power, imo.
I use it among many other things as a file launcher to send raw files or other formats with hot keys to Darktable, RawTherapee, Gimp of G’mic. You can find it here:
http://yap.bozart.eu/articles/gthumb/intro_en.html
(no ads, cookies or trackers, just the text).
In case you don’t read English, there’s also a Dutch version available. 
Regards, Paul.
