如何把水印加在中间

Question

     Graphics g = Graphics.FromImage( image );
                    g.DrawImage( copyImage , new Rectangle( image.Width - copyImage.Width , image.Height - copyImage.Height ,
                    copyImage.Width , copyImage.Height ) ,0 ,0 , copyImage.Width , copyImage.Height , GraphicsUnit.Pixel );
                    g.Dispose();
如何把水印加在中间?