如何以透明和全尺寸将一幅图像覆盖在另一幅图像上

Question

标题就是全部...

代码：

        Bitmap baseImage;
        Bitmap overlayImage;

        baseImage = (Bitmap)Image.FromFile(@"C:\temp\bg.png");

        overlayImage = (Bitmap)Image.FromFile(@"C:\temp\overlay.png");

        var finalImage = new Bitmap(overlayImage.Width, overlayImage.Height, PixelFormat.Format32bppArgb);
        var graphics = Graphics.FromImage(finalImage);
        graphics.CompositingMode = CompositingMode.SourceOver;

        graphics.DrawImage(baseImage, 0, 0);
        graphics.DrawImage(overlayImage, 0, 0);

        //show in a winform picturebox
        pictureBox1.Image = finalImage;

        //save the final composite image to disk
        finalImage.Save(@"final.png", ImageFormat.Png);

What happens

What i need

我不知道如何使叠加大小等于图像大小...