Can’t find the original thread but there was a discussion about image files not taking when an image is updated using the same file name as before. Response was that this should work. I have since realised that it does indeed work but the editor does not display the new image until one exits the module and restarts.
Ah, so it’s only a “semi-resolved” issue 
Since that behavior is still clearly a bug, just not quite as severe.
The solution would be to clear the old image from the image cache.
This looks similar to Bug 13794: vassalengine.org/tracker/sho … i?id=13794