- changed to default jpeg image for YouTubeVideo representation

- added blank line
- added small note to the docstring

IPython/lib/display.py

@@ -252,15 +252,19 class YouTubeVideo(IFrame):
 
     Other parameters can be provided as documented at
     https://developers.google.com/youtube/player_parameters#parameter-subheader
+    
+    When converting the notebook using nbconvert, a jpeg representation of the video
+    will be inserted in the document.
     """
 
     def __init__(self, id, width=400, height=300, **kwargs):
         self.id=id
         src = "https://www.youtube.com/embed/{0}".format(id)
         super(YouTubeVideo, self).__init__(src, width, height, **kwargs)
+    
     def _repr_jpeg_(self):
         from urllib import urlopen
-        return urlopen("https://img.youtube.com/vi/{id}/0.jpg".format(id=self.id)).read()
+        return urlopen("https://img.youtube.com/vi/{id}/default.jpg".format(id=self.id)).read()
 
 class VimeoVideo(IFrame):
     """