My new project deals with ARD-Mediathek. Not all videos have a 1x1 image, that could be used as cover thumbnail. And non square images do not please me as thumbnails, because of their aspect ratio is not being preserved ( see here Preserve aspect ratio for non square cover thumbnails ). If you could fix that, I would also use rectangular thumbnails, and consequently always have thumbnail and full cover urls.
Thumbnail is missing:
Rectangular Thumbnail is distorted:
