C# โปรแกรมจัดการเกี่ยวกับรูปภาพช่วยแนะนำวิธีการ crop ภาพ

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 TextBoxTest
{
    public partial class Form1 : Form
    {
        Point startPos;      // mouse-down position
        Point currentPos;    // current mouse position
        bool drawing;        // busy drawing

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            currentPos = Point.Empty;
            startPos =  Point.Empty;
            drawing = false;
        }

        private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
        {
            PictureBox pbImage = (PictureBox)sender;
            pbImage.Refresh();

            currentPos = e.Location;
            startPos = e.Location;
            drawing = true;
        }

        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            PictureBox pbImage = (PictureBox)sender;

            if (drawing)
            {
                currentPos = e.Location;
                pbImage.Refresh();         
            }
        }

        private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
        {
            PictureBox pbImage = (PictureBox)sender;

            if (drawing)
            {
                drawing = false;
                pbImage.Refresh();

                if (currentPos != startPos)
                    MessageBox.Show(string.Format("x: {0}, y: {1}, w: {2}, h: {3}",
                        Math.Min(startPos.X, currentPos.X), 
                        Math.Min(startPos.Y, currentPos.Y),
                        Math.Abs(startPos.X - currentPos.X),
                        Math.Abs(startPos.Y - currentPos.Y)), "Rectangle", MessageBoxButtons.OK);
            }
        }

        private void pictureBox1_Paint(object sender, PaintEventArgs e)
        {
            Rectangle rectangle = new Rectangle(
                Math.Min(startPos.X, currentPos.X),
                Math.Min(startPos.Y, currentPos.Y),
                Math.Abs(startPos.X - currentPos.X),
                Math.Abs(startPos.Y - currentPos.Y));

            using (Brush brush = new SolidBrush(Color.FromArgb(128, 100, 149, 237)))
            {
                if (startPos != Point.Empty)
                {
                    e.Graphics.DrawRectangle(Pens.CornflowerBlue, rectangle);
                    e.Graphics.FillRectangle(brush, rectangle);
                }

                if (drawing)
                {
                    e.Graphics.DrawRectangle(Pens.CornflowerBlue, rectangle);
                    e.Graphics.FillRectangle(brush, rectangle);
                }
            }
        }
    }
}