Operating system and graphics card
Linux Mint 18.3 Cinnammon (64 bit)
Nvidia Geforce GTX 1050 Ti
Broken: (example: 2.69.7 4b206af, see splash screen)
Short description of error
Gimp 2.10 has released a flatpak version of its program .
In User Preferences/ File/Edit Images Externally = flatpak cannot be launched.
For this reason a bug with Gimp developers was filed, please read the detailed description here:
Also, before filing the bug with Gimp I posted the issue on Blender Stack Exchange:
To summarize, Gimp developers believe that, as the flatpak can be launched via terminal, blender should have the option to launch a command line as well.
Exact steps for others to reproduce the error
Based on a (as simple as possible) attached .blend file with minimum amount of steps
Download Gimp 2.10 flatpak
Open Blender 2.79b
Set User Preferences/ File/Edit Images Externally/flatpak run org.gimp.GIMP
Unwrap your model
UV Map Pane Image/Edit Image Externally
Image editor could not be launched, please ensure that the path in User Preferences > File is valid, and Blender has rights to launch it
Please read the above links to both Gimp Developers and Blender Stack Exchange for further details about the testing done.