A feature that has been sorely needed on Viddler since day one has now finally made it out of cold storage and onto the main site. Using an image that you upload, rather than using a still frame from within the video, as the video's default thumbnail (or, the frame you see before you hit play) is now possible.
You can see the results on Eat Weird Episode 5. Gorgeous.
Also, this feature is exposed via the Viddler API's setThumbnail method.