This graphic is a little puzzle which I'm 'throwing out there' for discussion. It is a simple little cryptosystem. If you have any thoughts about what the system is, or how it might be broken, please comment. Even if your thought turns out to be wrong, it may spark an idea in another person.
My apologies that this puzzle, by its nature, is not accessible to people using screen readers or non graphical browsers.
This is not one of the regular weekly puzzles. A solution does exist, but I won't put it out there until I feel the time is right, which may be never!