Form1.cs

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

namespace DragSelectBox
{
    public partial class Form1 : Form
    {
        public bool leftMouseButtonIsDown;
        public Point leftMouseButtonDownLocation = Point.Empty;
        public Point currentMouseLocation = Point.Empty;

        public Form1()
        {
            InitializeComponent();
            pictureBox1.BackgroundImage = Image.FromFile(@"C:\Users\chris\Desktop\post\backgroundimage.jpg");
        }

        private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
        {
            leftMouseButtonIsDown = true;
            leftMouseButtonDownLocation = currentMouseLocation = e.Location;
            pictureBox1.Invalidate();
        }

        private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
        {
            leftMouseButtonIsDown = false;
            pictureBox1.Invalidate();
        }

        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            if(leftMouseButtonIsDown)
            {
                currentMouseLocation = e.Location;
                pictureBox1.Invalidate();
            }
        }

        private void pictureBox1_Paint(object sender, PaintEventArgs e)
        {
            if (leftMouseButtonIsDown)
            {
                Rectangle window = new Rectangle(
                        Math.Min(leftMouseButtonDownLocation.X, currentMouseLocation.X),
                        Math.Min(leftMouseButtonDownLocation.Y, currentMouseLocation.Y),
                        Math.Abs(leftMouseButtonDownLocation.X - currentMouseLocation.X),
                        Math.Abs(leftMouseButtonDownLocation.Y - currentMouseLocation.Y));

                Graphics G = e.Graphics;
                Pen pen = new Pen(Color.Red, 2);
                G.DrawRectangle(pen, window);
                pen.Dispose();
            }
        }
    }
}