As a public service announcement – if you need some software (on Linux but may well compile and run on other systems if they support Qt) to chop a big JPEG up into smaller files, I have written this (in my case to support building a test set for a neural network).
It’s at https://github.com/mcmenaminadrian/TestSetCreator and it’s GPL licenced (v 3).
cartesian product ... stuff about computing mostly 