<Canvas x:Name="ImageCanvas"
        Grid.Column="1"
        Background="Transparent">

    <Image x:Name="SelectedPartImage"
           Width="{Binding ElementName=ImageCanvas, Path=ActualWidth}"
           Height="{Binding ElementName=ImageCanvas, Path=ActualHeight}"
           Panel.ZIndex="0"
           MouseLeftButtonDown="OnMouseLeftButtonDown"
           Source="{Binding SelectedPartImageSource}" />

    <Ellipse x:Name="EllipseClick"
             Width="15"
             Height="15"
             Panel.ZIndex="1"
             Fill="{StaticResource GreenColor}"
             Visibility="Hidden" />
</Canvas>

// ActualWidth and ActualHeight are the values of the image shown.
RenderTargetBitmap rtb = new RenderTargetBitmap(
    (int)ActualWidth,
    (int)ActualHeight,
    96d,
    96d,
    System.Windows.Media.PixelFormats.Default);

rtb.Render(ImageCanvas);

PngBitmapEncoder pngEncoder = new PngBitmapEncoder();
pngEncoder.Frames.Add(BitmapFrame.Create(rtb));

System.IO.MemoryStream ms = new System.IO.MemoryStream();
pngEncoder.Save(ms);
ms.Close();

var bitmap = new BitmapImage();
bitmap.BeginInit();
bitmap.StreamSource = ms;
bitmap.CacheOption = BitmapCacheOption.OnLoad;
bitmap.EndInit();

System.IO.File.WriteAllBytes("test.png", ms.ToArray());

public void OnMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    DrawEllipse();

    Point point = Mouse.GetPosition(SelectedPartImage);

    if (!(point.ConvertWPFPointToImagePoint(
                  SelectedPartImageSource.PixelWidth,
                  SelectedPartImageSource.PixelHeight,
                  SelectedPartImage.ActualWidth,
                  SelectedPartImage.ActualHeight))
    {
        return;
    }

    SelectPositionSuccess = CheckAlpha((int)point.X, (int)point.Y);

    EllipseClick.Visibility = SelectPositionSuccess ? Visibility.Visible : Visibility.Hidden;

    if (SelectPositionSuccess)
    {
        //Saving the Canvas
        RenderTargetBitmap rtb = new RenderTargetBitmap(
            (int)ViewModel.SelectedPartImageSource.Width,
            (int)ViewModel.SelectedPartImageSource.Height,
            96d,
            96d,
            System.Windows.Media.PixelFormats.Default);

        rtb.Render(ImageCanvas);

        //endcode as PNG
        PngBitmapEncoder pngEncoder = new PngBitmapEncoder();
        pngEncoder.Frames.Add(BitmapFrame.Create(rtb));

        //save to memory stream
        System.IO.MemoryStream ms = new System.IO.MemoryStream();
        pngEncoder.Save(ms);
        ms.Close();

        var bitmap = new BitmapImage();
        bitmap.BeginInit();
        bitmap.StreamSource = ms;
        bitmap.CacheOption = BitmapCacheOption.OnLoad;
        bitmap.EndInit();

        System.IO.File.WriteAllBytes("test.png", ms.ToArray());
    }
}

public static Point ConvertWPFPointToDermaPoint(
    this Point inPoint,
    double inPixelWidth,
    double inPixelHeight,
    double inActualWidth,
    double inActualHeight)
{
    // Width Ratio
    inPoint.X *= inPixelWidth / inActualWidth;

    // X
    if ((int)inPoint.X > inPixelWidth - 1)
    {
        inPoint.X = inPixelWidth - 1;
    }
    else if (inPoint.X < 0)
    {
        inPoint.X = 0;
    }

    // Height Ratio
    inPoint.Y *= inPixelHeight / inActualHeight;

    // Y
    if ((int)inPoint.Y > inPixelHeight - 1)
    {
        inPoint.Y = inPixelHeight - 1;
    }
    else if (inPoint.Y < 0)
    {
        inPoint.Y = 0;
    }

    return new Point(inPoint.X, inPoint.Y);
}