When your Video message is sent to a customer you have two options for the thumbnail that they see in the email.

Choose this thumbnail from your app, or after filming on desktop. If you do not choose one, then we use the first frame of your video by default.

If you choose this option, we'll automatically make a Gif to go in your message email from the first few seconds of video. We suggest sending yourself a test if you use this as it can be hard to get correct...as Oli shows below

To choose these head to your account settings > Email customisation