Collapse the final level of an ImageDict to a list of images for side-by-side comparison

[melissaebrooks]

Simple ImageDict objects are easy to make, but making ones that use multiple images as the payload, for side-by-side comparisons, can be more tricky.

One simple way to do this is to create an ImageDict that references all available images, where the last level of the ImageDict selects between all of the images that you would like to be side-by-side. And this dictionary can have that level 'collapsed' and turned into a list of images (with the order set according to the sort order of the ImageDict).