Follow these steps to add a preview image for a video:
Add video element.
Double click on the video element.
In Settings, go to "Thumbnail".
Upload your image.

Publish or update your page.
Add video element.
Double click on the video element.
In Settings, go to "Thumbnail".
Upload your image.

Publish or update your page.
Published on: 02 / 06 / 2020