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?
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.