If you are not satisfied with the video thumbnail or cover showing by default you can add a custom thumbnail or custom cover to it. Let's have a look how to do it.
When a video is uploaded successfully as a cover video then at the bottom right corner of the video you will get an option "Upload Cover Image".
When you will click on that option it will give access to your computer and help you to find the desired cover you are looking for. Just select that image & click on OK to confirm.