Ahh, I see the issue is with the media:content enclosure , so I'll need to process it to amend their default output.
Now the problem is JW Player uses 720px wide thumbnails and you need 800px.
From your press release:
http://www.businesswire.com/news/home/2 ... -Publisher "
Roku Direct Publisher is also supported by top third-party OVPs, including Brightcove, JW Player, Kaltura and Ooyala"Would such support be a payable service with JW then? Because it doesn't seem to be particularly out of the box.