I wanted to learn a bit about the geojson format myself, so in playing around I made a version of the file you sent with the boundaries that have thumbnails.
To make this I ended up doing some combination of formulas in excel and cutting and pasting to make the geojson file from the data that was there. Then I imported the geojson into codap, exported that and pasted the new polygon features with added thumbnail over what was there in the csv.
(Bill I tried to join after importing the geojson but could not get that to work correctly.)
Attached is the geojson file I created as well.