I don't think it's something to do with MCEBuddy, see this:
http://community.mediabrowser.tv/permalinks/1413/how-do-i-configure-and-use-the-dvr-ms-metadata-plugin
http://community.mediabrowser.tv/permalinks/1413/how-do-i-configure-and-use-the-dvr-ms-metadata-plugin
The thumb image we extract is seemingly created when the video is first viewed in the Recorded TV section in windows media center. If the thumbnail is not showing in Media Browser try using Media Center to view the item in the Recorded TV section until the actual thumbnail appears.