YouTube: Thumbnail anzeigen per URL
http://img.youtube.com/vi/VIDEO-ID/default.jpg (Standard Größe)
http://img.youtube.com/vi/VIDEO-ID/mqdefault.jpg (Medium Quality Version)
http://img.youtube.com/vi/VIDEO-ID/hqdefault.jpg (High Quality Version)
http://img.youtube.com/vi/VIDEO-ID/maxresdefault.jpg (Maximale Auflösung)