We have already seen the testing of a new design for YouTube, one that looks more clean and modern–But more importantly is the fact that it fits with Google’s Material Design. Via a find in the YouTube support forum, we know of another update coming to YouTube Thumbnails.
Currently, in testing on the desktop, video thumbnails are now becoming GIF-like. This is on the homepage, search results, watch page, subscriptions tab, and trending tabs. The GIF-like animation comes in the form of a 3-second animation that will occur when you mouse over the thumbnail. This animation is only available on the desktop in Chrome versions 32 and up and Opera version 19 and up as of now.
Regarding which videos will get this treatment, they have to be at least over 30 seconds and will fit into certain topics that YouTube deems right. On the creator side, there is no way to activate this feature for a particular video as of yet. Hopefully, in the future, YouTube will give producers some control here, similar to how you can pick a thumbnail image.