Personally, I think your problem here is the number of videos all on one page...if I scroll REALLY QUICKLY down the page, and then go back up then I can see that some of the preview images have shown and some haven't.
But if I scroll slowly then they all load as I scroll.
I believe that the default 'preload' value is 'metadata' (so if it is missing then the browser should adhere to that) and that is enough to get the first frame to display it as the preview image. The other options are 'none' (which means no image) and 'auto' (which means the full video is auto-downloaded, ready to be played - you don't want to have that in effect on a page that has so many videos either)