Thumbnails give a preview of content or attract further engagement if it's associated with a call-to-action.
By default the first frame of your video is set as its thumbnail. You can change this to a different frame within the video or another image.
Change the thumbnail for a video tile:
Navigate to and select the video in the site editor;
In the left editing pane, under Thumbnail, select from three options to change the video thumbnail:
Default from video - the default frame of the video, usually the first frame;
Custom Image - your preferred image, you can either upload an image or get it from the Media Library;
Frame from video - select a frame from the uploaded video.
Once the image or frame is selected, it will let you crop the image to fit in the tile