Preview thumbnails are not generated for videos that are uploaded. Therefore, using the Atom Publishing Protocol, also known as AtomPub, you can associate a thumbnail preview image with a video. You may not use this to update the preview image associated with a photo.
You can derive the URL from the video document entry. Use the web address specified as the "href" attribute of the "<link rel="http://www.ibm.com/xmlns/prod/sn/cmis/multipart-form/object">" element.