I should have mentionned I did write a script for source video histogram with Avisynth:
https://github.com/butterw/bShaders/...aHistogram.avs
It does also work with images, but I'm not convinced it's the right tool for the job.
I'm hoping there is some free GUI tool (based on one of the image libraries), that can handle all basic needs.
Avisynth + GUI: AvsPmod
Issues: language/syntax limitations, no command line text output.
I was trying to do a simple script to combine 2 png screenshots for split screen comparison. IIRC, the issue was handling user input, errors and output filename...
Vapoursynth might do better.