Every so often you will come across a video that has a pretty rubbish thumbnail. Most of the time a video thumbnail in Quicklook will show the first frame. But you can easily change the thumbnail with only the couple of steps. For this example (which isn’t the best) I am going to use Experiment Of The Week, which is a great video if you want to subscribe.
The first step is to locate your file. If you set Quicklook in Finder you will see you video displayed with the thumbnail.
This will paste the frame into the thumbnail of the video. It will then be used for every thumbnail shown. The quality of the thumbnail will depend on the image you chose. You can also images from other sources.