用C#在form里画透明、不透明图像

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

namespace graphic
{
    public partial class Form1 : Form
    {
        Image MyPNG = Image.FromFile("E:\\Image\\png\\a.png");
        Image MyJPG = Image.FromFile("E:\\Image\\png\\a.jpg");
        Graphics PanelCGI;
        public Form1()
        {
            InitializeComponent();
            PanelCGI = this.panel1.CreateGraphics();
        }
         private void button1_Click(object sender, EventArgs e)
        {
            PanelCGI.Clear(Color.LightGray);
            PanelCGI.DrawImage(MyPNG, 20, 20);

        }

        private void button2_Click(object sender, EventArgs e)
        {
            PanelCGI.Clear(Color.LightGray);
            PanelCGI.DrawImage(MyJPG, 20, 20);
        }

    }
}