You can upload custom thumbnails to any of your videos. The advantage of a custom thumbnail is that you have creative control over what users see as your thumbnail and that it is not limited to stills from the video itself (though it has to be relevant and accurately representative of the content in the video).
To upload a custom thumbnail you can follow the below steps:
1- Log onto your account
2- Click on "video manager" (in the drop down menu in the upper right hand corner)
3- Click on "edit" by the video you would like to add a thumbnail for
4- Click on "upload thumbnail"
The newest version of our Creator Playbook includes a couple of pages (please see pages 44-46) that further explain the feature. I would highly recommend that resource as a great starting point on how to improve channel and video performance.