如何绘制多个矩形并将其保存到已加载的.jpg C＃

Question

我试图在Windows窗体应用程序中打开一个.jpg，在其上绘制一个矩形并用矩形保存。 在下面的代码中，我可以加载.jpg并在pictureBox中绘制一个矩形，但不能用矩形保存.jpg。有人知道这个问题吗？

    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 EditScreen
    {
    public partial class Form1 : Form
    {

    Rectangle mRect;
    Image mainimage;
    Bitmap newBitmap;
    Graphics g;
    Boolean opened = false;
    OpenFileDialog ofd = new OpenFileDialog();
    SaveFileDialog sfd = new SaveFileDialog();
    public Form1()
    {
        InitializeComponent();
        this.DoubleBuffered = true;
    }

    private void button1_Click(object sender, EventArgs e)
    {
        DialogResult dr = ofd.ShowDialog();
        if (dr == DialogResult.OK)
        {
            mainimage = Image.FromFile(ofd.FileName);
            newBitmap = new Bitmap(ofd.FileName);
            pictureBox1.Image = mainimage;
            opened = true;  
        }
    }

    private void button2_Click(object sender, EventArgs e)
    {
        DialogResult dr = sfd.ShowDialog();
        if (opened)
        {
           mainimage.Save(sfd.FileName);
        }
    }

    private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
    {
        mRect = new Rectangle(e.X, e.Y, 0, 0);
        pictureBox1.Invalidate();
    }

    private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left)
        {
    mRect = new Rectangle(mRect.Left, mRect.Top, e.X - mRect.Left, e.Y - mRect.Top);                               
            pictureBox1.Invalidate();
        }
    }

    private void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        using (Pen pen = new Pen(Color.Red, 2))
        {
            Graphics g = e.Graphics;
            g.DrawRectangle(pen, mRect);
        }
    }
}
}

我听说我必须在位图上绘画，但是当我这样做时，我不知道如何使用PaintEventArgs。你能给我一个代码来举例来保存它吗？