Lightweight script and documentation to clean up image testing enviro…

[bnlawrence]

This is a temporary fix for #135 ... basically one needs to be able to say "ok, I've looked at all these test failures" and they're all noise, replace the source images with mine, so we don't have failures.

[sadielbartholomew]

I might keep some of the convenience functions here, which are useful in some senses, though for a more general solution to this, at least the first step in a plan for one, I prefer the method outlined here in my PR #139. Apologies that most of the work towards that was local until now - had to do the release first before I came back to it.