请各位帮忙解决一下~我的这代码是用窗体显示图片```可图片出不来~是什么原因呢？

/**
 * @(#)tt.java
 *
 *
 * @author
 * @version 1.00 2008/4/6
 */
import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
public class tt extends Frame {
        
    /**
     * Creates a new instance of <code>tt</code>.
     */
    Image img;
    public tt() {
        
        this.setLayout(new FlowLayout());
        this.setSize(720,480);
        this.addWindowListener(new a());
        //this.show();
        this.setVisible(true);
    }
    public void Paint(Graphics g)
    {
        Toolkit tk=Toolkit.getDefaultToolkit();
        Image img=tk.getImage("1.jpg");
        g.drawImage(img,0,20,this);
    }
    public void update(Graphics g)
    {
        this.Paint(g);
    }
   
    public void repaint(Graphics g)
    {
        this.Paint(g);
    }
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        new tt();
    }
}
class a extends WindowAdapter
{
    public void windowClosing(WindowEvent e)
    {
        System.exit(1);
    }
}