When you upload a video, Fuse extracts the first frame of the video and uses this as the thumbnail. This means that if the first frame in your the video is a blank screen, such as a white or black frame, this will be used as the thumbnail.
If you wish to change find that the automatically extracted thumbnail is unsuitable after the video has been uploaded, you can come back and upload your own one using the Add thumbnail option on the Edit content screen for the video.
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