این لینک رو بررسی کنید.

این لینک هم هست

دستتون درد نکنه 

private void button1_Click(object sender, EventArgs e)
        {
            Bitmap YourLogo = new Bitmap(@"D:\photo_2016-02-20_18-08-34-300x300.jpg");
            Bitmap YourPicture = new Bitmap(@"D:\sucker_punch_samurai-wallpaper-1600x900.jpg");
            YourPicture = WatermarkImage(YourPicture, YourLogo);
            YourPicture.Save(@"D:/watermarkedImage.png");
        }
        public static Bitmap WatermarkImage(Bitmap image, Bitmap watermark)
        {
            using (Graphics imageGraphics = Graphics.FromImage(image))
            {
                watermark.SetResolution(imageGraphics.DpiX, imageGraphics.DpiY);

                int x = (image.Width - watermark.Width) /1500;
                int y = (image.Height - watermark.Height) /700;

                imageGraphics.DrawImage(watermark, x, y, watermark.Width, watermark.Height);
            }

            return image;
        }

 سورس اضافه کردن لوگو 

نتیجه