Last tested: Aug 21, 2017
Image downloads are based on Production. It's is possible that you are in Development Mode.
While in Dev Mode, you may have saved a Look or Explore and then you defined at least one dimension or measure, BUT, you have yet to deploy to production. Therefore, while in Dev Mode and without a deployment to production, your Looks do not yet exist and are not available to download as a PNG/PDF.
This is usually fixed with deploying to production and then reattempting the download.