java awt 设置为背景图不能显示

我用java awt编写的主界面显示背景图片，但是，却显示不出来，请各位高手帮忙解决以下了：谢谢！

程序如下：
import java.awt.Canvas;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.Toolkit;
import javax.swing.ImageIcon;
import javax.swing.JLabel;
import javax.swing.JPanel;
import java.awt.*;
import javax.swing.*;
class MyJFrame extends JFrame
{
private static final long serialVersionUID = 1L;
private JPanel Content;
MyJFrame()
{
Content=(JPanel)getContentPane();
PictureJPanel myjpanel=new PictureJPanel();
Content.setOpaque(false); //设内容窗格为透明。。
Content.setLayout(new FlowLayout());

Content.add(myjpanel,-1);
Content.add(new JButton("确认"),0);
this.getLayeredPane().setLayout(null);
this.getLayeredPane().add(myjpanel,new Integer(Integer.MIN_VALUE));

setBounds(100,100,800,600);

setVisible(true);
validate();
}

}
class PictureJPanel extends JPanel
{
private static final long serialVersionUID =2L;
public PictureJPanel()
{
//把图片显示在标签里。。。
JLabel pictureLabel = new JLabel();
pictureLabel.setIcon(new ImageIcon("25.BMP"));
pictureLabel.setBounds(0,0,800,600);



setOpaque(false);
add(pictureLabel);
}
}
public class MainFrame
{
public static void main(String args[])
{
MyJFrame myjframe=new MyJFrame();
//new WindowMain("冠心病诊治规律及综合治疗方案研究");

//MyFrame.add(canvas2,BorderLayout.CENTER);
myjframe.setDefaultCloseOperation(myjframe.EXIT_ON_CLOSE);
myjframe.setVisible(true);
myjframe.validate();

}
}