draw an a square on image in picturebox

  using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace RedDotAnnotate
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        Bitmap OriginalImage;
        Point myPts = Point.Empty;

        private void button1_Click(object sender, EventArgs e)
        {
            //Open File Dialog to load image files
            openFileDialog1.FileName = "";
            openFileDialog1.Title = "Images";

            //Filter the filedialog, so that it will show only the mentioned format images
            openFileDialog1.Filter = "PNG Image(*.png)|*.png|JPG Image(*.jpg)|*.jpg|BMP Image(*.bmp)|*.bmp";
            openFileDialog1.ShowDialog();

            if (openFileDialog1.FileName.ToString() != "")
            {
                pictureBox1.ImageLocation = openFileDialog1.FileName.ToString();
                OriginalImage = new Bitmap(openFileDialog1.FileName.ToString());
            }

            // butSave.Enabled = true;
        }

        private void pictureBox1_mouseDown(object sender, MouseEventArgs e)
        {
            Point myPts = new Point(e.X, e.Y);
            Bitmap b = (Bitmap)pictureBox1.Image;
            Graphics g = Graphics.FromImage(b);
           // myPts.add(new Point(e.X, e.Y));
            Invalidate();

        }
       // List myPts = new List();

        private void pictureBox1_Paint(object sender, PaintEventArgs e)
        {
           
            Graphics g = e.Graphics;
            foreach (Point p in myPts)
            {
                g.DrawEllipse(new Pen(Color.Green), p.X, p.Y, 10, 10);

            }            

        } 
    
        
    }

  }