Silverlight: Define image source as external website

public class UserPhotoConverter : IValueConverter
{
     public UserPhotoConverter() { }

     #region IValueConverter Members
     public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
     {
          if (value == null) return "";

          MemoryStream stream = new MemoryStream((byte[])value);
          BitmapImage img = new BitmapImage();
          img.SetSource(stream);
          return img;
     }


     public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
     {
          throw new NotImplementedException();
     }

     #endregion
}