Main方法：
static void Main(string[] args)
        {
            var dateNow = DateTime.Now;
            string saveFile = Environment.CurrentDirectory+"\\puctures\\" + dateNow.Year.ToString() + (dateNow.Month).ToString() + dateNow.Day.ToString() + dateNow.Hour.ToString() + dateNow.Minute.ToString() + dateNow.Second.ToString()+".jpg";
            string imgBackPath = @"D:\Learning\PracticeDemo\PracticeDemo\images\is407-046.jpg";
            string imgBackExtension = Path.GetExtension(imgBackPath);
            
            string imgPath = @"D:\Learning\PracticeDemo\PracticeDemo\images\is104-052.jpg";
            string imgExtension = Path.GetExtension(imgPath);

            if (File.Exists(imgBackPath) == false||File.Exists(imgPath)==false||
                (
                imgBackExtension!=".gif"&&
                imgBackExtension!=".png"&&
                imgBackExtension!=".jpg"
                )||(
                imgBackExtension != ".gif" &&
                imgBackExtension != ".png" &&
                imgBackExtension != ".jpg"
                )
                )
            {
                Console.WriteLine("文件不存在或不支持该扩展名！");

                Console.WriteLine("End"); 
            }
            Image imgBack = Image.FromFile(imgBackPath);
            Image img = Image.FromFile(imgPath);

            Bitmap bmp = MergeImgHelp.CombinImage(imgBack, img);
            MemoryStream ms = new MemoryStream();
            bmp.Save(saveFile, ImageFormat.Jpeg);

            Console.WriteLine("文件保存成功！");
            Console.ReadKey();
        }

public class MergeImgHelp
    {
        /// <summary>
        /// 合并图片
        /// </summary>
        /// <param name="imgBack"></param>
        /// <param name="img"></param>
        /// <param name="xDeviation"></param>
        /// <param name="yDeviation"></param>
        /// <returns></returns>
        public static Bitmap CombinImage(Image imgBack, Image img, int xDeviation = 0, int yDeviation = 0)
        {
            Bitmap bmp = new Bitmap(imgBack.Width, imgBack.Height);

            Graphics g = Graphics.FromImage(bmp);
            g.Clear(Color.White);
            g.DrawImage(imgBack, 0, 0, imgBack.Width, imgBack.Height);

            g.DrawImage(img, imgBack.Width / 2 - img.Width / 2 + xDeviation, imgBack.Height / 2 - img.Height / 2+yDeviation, img.Width, img.Height);
            GC.Collect();
            return bmp;
        }
    }