Curious about the same question. The sho.co site
At the moment you cannot select the video thumbnail manually. It has been suggested as a feature though and if you would like to see it implemented like the original feature request post. The more likes the more seriously we will take it.
Link to "suggested as a feature" takes me back here
Link amended, sorry!
Jerry Horwood
When I publish a Scribe and then look at it online, the first screen I see before I play the video is the final scene. This 'gives the game away' if you want to build a story. How do I change the thumbnail to show the first screen rather than the last?