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 video tile thumbnail from the Site Editor
Navigate to and select the video tile 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
Change the video thumbnail from the Media Library
Go to the Media Library
Look for the video that needs thumbnail change and select it
Click Change Image in the right editing pane
Select the thumbnail from the video frames available