ペイント超簡易版

MinimunPaint.cs

using System.Drawing;
using System.Windows.Forms;

namespace WindowsFormsApp1
{
    public partial class Form1 : Form
    {
        Point posPrevious;

        public Form1()
        {
            InitializeComponent();

            Bitmap bmp = new Bitmap(pictureBox1.ClientSize.Width, pictureBox1.ClientSize.Height);

            using (Graphics g = Graphics.FromImage(bmp))
            {
                g.Clear(Color.White);
            }

            pictureBox1.BackgroundImage = bmp;
        }

        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left || e.Button == MouseButtons.Right)
            {
                using (var front = pictureBox1.CreateGraphics())
                {
                    if (e.Button == MouseButtons.Left)
                    {
                        front.DrawLine(Pens.Black, posPrevious, e.Location);
                    }
                    else if (e.Button == MouseButtons.Right)
                    {
                        front.FillEllipse(Brushes.White, posPrevious.X - 5, posPrevious.Y - 5, 10, 10);
                        front.FillEllipse(Brushes.White, e.Location.X - 5, e.Location.Y - 5, 10, 10);
                        front.DrawLine(new Pen(Color.White, 10), posPrevious, e.Location);
                    }
                }

                posPrevious = e.Location;
            }
        }

        private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
        {
            posPrevious = e.Location;
        }
    }
}