Fetching thumbnail for a stream or video, via browser, with helix API?

So this means the user needs to be logged in before seeing the thumbnail? What would the URL I should be fetching once I have the oauth token to get the thumbnail for a specific video?

BTW I had been hoping for something as simple as Vimeo: http://vimeo.com/api/v2/video/xxxxx.json , where xxxxx is the video id.