Load https://musicbrainz.org/release/c5355b3d-3f53-4388-90c5-f51e40182462?tport=8000, save it, all with the config from PICARD-1001 (I hope I really haven't changed anything). Note that the 22kB 340px x 340px image is the second one in the album info dialog and is the one saved as front.png, the large png is used as folder.png. So
image #1 -> folder.png
image #2 -> front.png
Make sure 'replace front cover art on drop' is selected. Drop a different PNG file (for simplicity) on the cover art, open the album info dialog. The 22kB png is still there, but it's now image #1, the new one is image #2. Now save this.
image #1 (22kB) -> front.png
image #2 (the new one) -> folder.png