Hi,

Loading from generated thumbnails using vtt is the popular way to do it. I haven’t seen any other player which automatically generates thumbnails. I imagine it would create server overheads opposed to just delivering a small thumbnail. You normally only have 1 thumbnail every minute or so. Plus you would need the flash fallback anyway.

The JW player currently offers this feature. It works with flash, html and rtmp..

http://support.jwplayer.com/customer/portal/articles/1407439-adding-preview-thumbnails