This does not stop the process of uploading your images and videos to the site in order for your clinician to review and evaluate. This is currently just an aesthetic problem. If you tap on the video or image the thumbnails will return to normal.
The videos that have a black thumbnail will also have a time stamp of 00:00:00 on it instead of how long the video is. This too is currently just an aesthetic problem and will have no effect on your videos uploading or the quality they will have once sent to your account.