JPanel不显示背景图像

这是一个非常简单的程序，我已经尽力了，但是JPanel没有提供背景图像.我只希望面板上有一个简单的背景图像.

It is very simple program and i have tried my best but the JPanel doesn't come up with a background image. I just want a simple background image on my panel.

这是我的代码:

import javax.swing.*;
import java.awt.image.BufferedImage;
import javax.imageio.ImageIO;
import java.io.*;
import java.awt.Graphics;

class PanelEx extends JPanel
{
    BufferedImage img;

    PanelEx()
    {
        try
        {
            img = ImageIO.read(new File("C:/Users/Pictures/s_4261.jpg"));
        }
        catch(Exception ex)
        {
            ex.printStackTrace();
        }
    }
    public void printComponent(Graphics g)
    {
        g.drawImage(img,0,0,null);
    }
}

class JPanelEx2 extends JFrame
{
    PanelEx pe;

    public static void main(String args[])
    {
        new JPanelEx2();
    }

    JPanelEx2()
    {
        pe = new PanelEx();

        add(pe);
        setTitle("JPanel Title");
        setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        setSize(320,240);
        setVisible(true);
    }
}

预先感谢