Hi! ^^ I've moved your thread to the Branding and Channel Design forum. =)
I think the most important thing is to keep thumbnails clutter-free but intriguing. No one is going to take time out of their day to grab a magnifying glass and read lots of little text on a thumbnail, but they will click if it looks like a welcoming and interesting picture. ^^ I don't think they need to all match, but they need to be intriguing. =)