How are video thumbnails chosen?
There are many ways to skin this cat. I'll explain one way that I know is used in industry.


Create candidate frames algorithmically using the image features; e.g., the histogram, sharpness, presence of humans, faces, and anything else you can think of.
Serve different candidates to different users and see which ones are popular
You can stop once the popularity contest results stabilize. However, for bonus points you can learn the ranking function that yielded the list based on the features enumerated in part one.
Use the ranking function to select the best frame, or cull the list of candidates for subsequent tests.