// I pass in 320x150 or 434x210 or 558x270 depending on the scale.
public static MemoryStream ToPng(
    this FrameworkElement frameworkElement,
    double width,
    double height)
{
    BitmapSource bitmapSource = ToBitmapSource(frameworkElement, width, height);

    PngBitmapEncoder pngBitmapEncoder = new PngBitmapEncoder();
    pngBitmapEncoder.Frames.Add(BitmapFrame.Create(bitmapSource));

    MemoryStream memoryStream = new MemoryStream();
    pngBitmapEncoder.Save(memoryStream);

    memoryStream.Position = 0;

    return memoryStream;
}

public static BitmapSource ToBitmapSource(
    this FrameworkElement frameworkElement,
    double width,
    double height)
{
    Size renderingSize = new Size(width, height);
    frameworkElement.Measure(renderingSize);
    Rect renderingRectangle = new Rect(new Point(0, 0), renderingSize);
    frameworkElement.Arrange(renderingRectangle);
    frameworkElement.UpdateLayout();

    Rect bounds = VisualTreeHelper.GetDescendantBounds(frameworkElement);
    RenderTargetBitmap renderBitmap = new RenderTargetBitmap(
        (int)frameworkElement.ActualWidth,
        (int)frameworkElement.ActualHeight,
        96,
        96,
        PixelFormats.Pbgra32);

    DrawingVisual drawingVisual = new DrawingVisual();

    using (DrawingContext drawingContext = drawingVisual.RenderOpen())
    {
        VisualBrush visualBrush = new VisualBrush(frameworkElement);
        drawingContext.DrawRectangle(visualBrush, null, new Rect(new Point(), bounds.Size));
    }

    renderBitmap.Render(drawingVisual);

    return renderBitmap;
}