There isn't really many choices: darktable and RawTherapee seem to be the most mature software for this. If you're looking for an easy way, just use JPEG and edit them software like GIMP. This works fine in most casual/hobby situations. Otherwise, you might as well invest some time to learn more specialized software.
Yes, OP knows it's not FOSS, it said so in the title and in their comment... It's free as in free beer (gratis), not as in freedom (libre). The same goes for FDRTools (which is in the sidebar), as well as the powerful image viewer IrfanView. However, there's nothing wrong with discussing tools like these, so long as it's made clear which category it falls into.
ImageMagick (through the convert
tool) can do some creative things to compress video gifs, including delta compression using transparency, dithering, etc. Not all of it is suited for automated scripts, though, as sometimes it's a judgment call when it comes to resultant quality.
Here is one tutorial, although there are more out there if you want to look for them.
Whilst theoretically it's best to stick to the official repos, it's fine to use PPAs in my personal experience.
A couple you might want to look into would be PPAs like ppa:pmjdebruijn/darktable-release for the latest Darktable releases, and for a large collection of niche applications (whether Luminance for creating HDR images or Macrofusion for combining rack focus shots), ppa:dhor/myway
The GIMP included in the official repos is the latest version, and it'll be a while before 2.10.
The downloads will show up at https://www.gimp.org/downloads/devel/ in a more dedicated way, making it obvious that the 2.9.x are development releases.
For the moment, this still redirects to /downloads/
I am not sure if that is possible in digikam. If they are exact duplicates (including the metadata and file size), you can use another software that compares hashes to find and delete them, like FSlint (http://www.pixelbeat.org/fslint/).
One solution might be to use Redshift as described in this stackexchange post.
Another would probably be to create a .icc profile for the monitor in question which defines the color temp you want, and then assign it from gnome-color-panel (or whatever environment you work under) - however, I haven't had luck creating a simple color temp changing profile in dispcalgui in the five minutes I've tried and I can't spare more time right now (I'm no expert on these things).
I remembered that I've used Cinepaint a few times. It's a fork of an older GIMP version but it will edit at 16 and 32-bits per channel unlike GIMP which is 8-bit only (for now but changing with GEGL)
Perhaps as an alternative to digikam: Geeqie is super fast and groups image and sidecar files together. It's my favorite tool, when browsing through a larger stack of images.
I installed Geeqie from the Linux Mint Software Manager and the version currently available is 1.4. I tried downloading the latest version from http://www.geeqie.org/ by selecting "Linux Mint" from the distro selection box and I'm taken to
https://community.linuxmint.com/software/view/gqview
I then click on the "Install" button and a pop-up is displayed that says "Package 'gqview' is virtual and it stops right there. Is there another way I can install 1.5 on Linux Mint?
TIA!
Um, looks a tad small to be packing any reasonable amount of storage. Unless you want to shell out for something like this, it's not going to be very pocketable.