You can select a custom thumbnail for any video by navigating to the Media Settings pane. Just click the Media options icon for any video, then click "Settings."
From the Media Settings window, as the asset owner or a Media Library admin, you'll be able to select a new thumbnail for your video. Just click the image in the upper-right of the screen.
You'll now see thumbnails to choose from. To change the thumbnail for your video, simply click on the image you want as the thumbnail.
You'll also see a plus (+) icon, which you can click if you'd like to upload a custom thumbnail.
Your new thumbnail will take the place of the old thumbnail. Click "Change" to save.
Now you'll see the new thumbnail on the Media Info page, as well as in the Media Library view, and when the video is embedded.