silverlight: 如何获取流媒体的第一帧

<StackPanel Width="600">
<MediaElement x:Name="ME" MouseLeftButtonDown="ME_MouseLeftButtonDown"/>
<telerik:RadWrapPanel x:Name="ThumbnailsPanel"/>
</StackPanel>

private void ME_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
WriteableBitmap bitmap = new WriteableBitmap(ME, new TranslateTransform());
Image img = new Image();
img.Width = 140;
img.Margin = new Thickness(5);
img.Source = bitmap;
this.ThumbnailsPanel.Children.Add(img);
}