You can easily take screenshots and capture them as frames from video assets and use them as thumbnails.
Steps to add thumbnails from a video using Capture button are mentioned below:
A. Play the video asset and click on the Screenshot icon to grab the screenshot of the frame you wish to add as keyframe.
B. In the frame capture window, click on Capture button.
C. Keyframe updated successfully message appears after the keyframe is added to either Managed storage location or Cloud storage based on the configuration.
D. Click on the same asset in the Browse widget to refresh and update the changes made.
E. In the Keyframes widget, new keyframe gets displayed. Click and select the thumbnail image which needs to be kept as default thumbnail. Select Public (visible to all users) or Private (visible only to this user) radial buttons and then click on Make this as Default button.