public void SetThumbnail( 
   BitmapSource thumbnail
)

public Task SetThumbnailAsync(Bookmark bookmark, string imagePath)
{
  //Set the thumbnail to an image on disk, ie. C:\Pictures\MyPicture.png.
  BitmapImage image = new BitmapImage(new Uri(imagePath, UriKind.RelativeOrAbsolute));
  return QueuedTask.Run(() => bookmark.SetThumbnail(image));
}