Have you wondered about how Google pick thumbnails for YouTube videos you uploaded? Well, I have. it seems like pretty random and people online had different theories posted. So I spent a few minutes to finally tracked down some current info. Have a read of Google talking about “Smart Thumbnails on YouTube” to understand how Google’s algorithm seems to randomly picked thumbnails for your YouTube videos, but it actually tries to do it smartly to avoid problems its previous method lead to.
Also check out Google’s blog entries tagged with YouTube and Video.