方法1，使用自带的画刷进行绘制水印

示例代码

 public partial class Form1 : Form
 {
     public Form1()
     {
         InitializeComponent();
     }
     string photoPath=string.Empty;
     Bitmap image = null;
     private void button1_Click(object sender, EventArgs e) //选择照片
     {
         OpenFileDialog dialog = new OpenFileDialog(); //打开对话框
         dialog.Filter = "|*.jpg";  //打开文件过滤器
         if (dialog.ShowDialog() == DialogResult.OK)
         {
             photoPath = dialog.FileName; //拿到用户选择文件的全路径
             pictureBox1.Image = System.Drawing.Image.FromFile(photoPath) ;  //显示照片   
         }
     }
    
     private void button2_Click(object sender, EventArgs e) //添加水印
     {
         image = new Bitmap(photoPath);
         Graphics graphics = Graphics.FromImage(image);

         string watermarkText = "测试水印"+DateTime.Now;
         Font font = new Font("宋体", 12);
         Brush brush = new SolidBrush(Color.Red);
         Point position = new Point(10, 10); //显示的初始位置

         graphics.DrawString(watermarkText, font, brush, position);
         graphics.Dispose(); //释放资源
         pictureBox1.Image = image;
         
     }

     private void button3_Click(object sender, EventArgs e) //保存带水印照片
     {
         SaveFileDialog saveFileDialog = new SaveFileDialog(); //保存文件对话框
         saveFileDialog.Filter = "照片|*.jpg";    //保存的文件格式
         if (saveFileDialog.ShowDialog() == DialogResult.OK)
         {
             string filePath = saveFileDialog.FileName;   // 获取用户选择的文件全路径
             image.Save(filePath);
         }
     }
 }

前台界面