关于BufferedImage编辑图片的问题

我下面的代码执行后，所载的图是个黑的，不过字可以显示

Java code
package test_img;

import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.ByteArrayOutputStream;
import java.io.File;

import javax.imageio.ImageIO;

import com.sun.image.codec.jpeg.JPEGImageEncoder;

import sun.awt.image.ToolkitImage;

class GuiCamera
{
private String fileName; // 文件的前缀

private String defaultName = "GuiCamera";

static int serialNum = 0;

private String imageFormat; // 图像文件的格式

private String defaultImageFormat = "jpg";

// Dimension d = Toolkit.getDefaultToolkit().getScreenSize();


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;
}


public void snapShot()
{
try
{

Toolkit tk = Toolkit.getDefaultToolkit();
Image img = tk.getImage(GuiCamera.class.getClassLoader().getResource("img\\demo1.jpg"));
BufferedImage screenshot = new BufferedImage(480,360,BufferedImage.TYPE_3BYTE_BGR);
Graphics g = screenshot.getGraphics();
g.drawOval(20, 20, 20, 20);
g.drawImage(img, 0,0, null);
g.drawString("cut image", 50, 50);
screenshot = screenshot.getSubimage(0, 0, 400, 300);
g.dispose();
screenshot.flush();
serialNum++;
// 根据文件前缀变量和文件格式变量，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! ");
} catch (Exception ex)
{
System.out.println(ex);
}
}

public static void main(String[] args)
{
GuiCamera cam = new GuiCamera("Test", "jpg");
cam.snapShot();
}
}