When I Google for one of my videos, it shows up at the top with a preview image. One of my other videos just shows up without a preview image. I think it's because that video does not have enough thumbs up ratings. Is that correct? How many are needed before it appears in Google with a preview?
How to make YouTube video appear in Google
google;youtube
Nobody knows how exactly Google is indexing the web and their YouTube, but I'm pretty sure that thumbs up ratings have nothing to do with the preview image. Give it a few days and try again.