When you upload a new video to your Asset Library, by default the first frame is chosen as a thumbnail. It might happen that the thumbnail is a black frame. If you have a series of black thumbnails, it hard to recognize a specific video. That is why, you might want to edit the thumbnail to be more meaningful and recognizable for users.
Click the asset you want to update.
Open the Edit tab.
Expand the Generate new thumbs tab.
Select the frame you prefer as a thumbnail with your mouse and click Current video position.
The new thumbnail will be generated within a few minutes.
You can also automate the video thumbnail generation so that you do not have to manually change it for every video. For example, you can decide that a thumbnail should be created from the screen that appears after 4 seconds of the video have passed.
Contact your Customer Success Manger if you would like to have this feature enabled for your portal.