import java.awt.Dimension; 
import java.awt.Rectangle; 
import java.awt.Robot; 
import java.awt.Toolkit; 
import java.awt.image.BufferedImage; 
import java.io.File; 

import javax.imageio.ImageIO; 

/******************************************************************* 
 * 本JavaBean可以直接在其他Java应用程序中调用，实现屏幕的"拍照" 
 * This JavaBean is used to snapshot the GUI in a  
 * Java application! You can embeded 
 * it in to your java application source code, and us 
 * it to snapshot the right GUI of the application 
 * @see javax.ImageIO 
 * @author linys([email]tolys@126.com[/email]) 
 * @version 1.0 
 * 
 *****************************************************/ 

public class GuiCamera 
{   
    private String fileName; //文件的名字
    private String defaultName = "GuiCamera"; 
    static int serialNum=0; 
    private String imageFormat; //图像文件的格式 
     private String defaultImageFormat="png"; 
    Dimension d = Toolkit.getDefaultToolkit().getScreenSize(); 

    /**************************************************************** 
     * 默认的文件名字为GuiCamera，文件格式为PNG格式 
     * The default construct will use the default  
     * Image file surname "GuiCamera",  
     * and default image format "png" 
     ****************************************************************/ 
    public GuiCamera() { 
      fileName = defaultName; 
      imageFormat=defaultImageFormat; 
     
    } 

    /**************************************************************** 
     * @param s the surname of the snapshot file 
     * @param format the format of the  image file,  
     * it can be "jpg" or "png" 
     * 本构造支持JPG和PNG文件的存储
     ****************************************************************/ 
    public GuiCamera(String s,String format) { 
     
      fileName = s; 
      imageFormat = format; 
    } 
     
    /**************************************************************** 
     * 对屏幕实行拍照 
     * snapShot the Gui once 
     ****************************************************************/ 
    public void snapShot() { 
     
      try { 
      //拷贝屏幕到一个BufferedImage对象screenshot 
        BufferedImage screenshot = (new Robot()).createScreenCapture(new 
            Rectangle(0, 0, (int) d.getWidth(), (int) d.getHeight())); 
        serialNum++; 
        //根据文件前缀和文件格式，自动生成文件名 
        String name=fileName+String.valueOf(serialNum)+"."+imageFormat; 
        File f = new File(name); 
        System.out.print("Save File "+name); 
      //将screenshot对象写入图像文件 
        ImageIO.write(screenshot, imageFormat, f); 
        System.out.print("..Finished!\n"); 
      } 
      catch (Exception ex) { 
        System.out.println(ex); 
      } 
    } 

    public static void main(String[] args) 
    { 
        GuiCamera cam= new GuiCamera("d:\\Hello", "jpg");//定义文件格式
        cam.snapShot(); 
    } 
}

------解决方案--------------------
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagLayout;
import java.awt.Image;
import java.awt.Toolkit;

import javax.swing.JPanel;



public class MyPanel extends JPanel {
	
	private static final long serialVersionUID = 1L;
	private Image img = Toolkit.getDefaultToolkit().getImage("filename");