title: How to get Stream from ImageSource object for saving in Pictures Library? (Windows Phone 7, C#)
body: <p>I have an <code>ImageSource</code> object (or possibly a <code>BitmapImage</code> object). I want to save this to the Pictures Library, using the <code>SavePicture</code> method of <code>MediaLibrary</code> class (XNA framework). How can I achieve this? 
  Thanks in advance.</p>