有没有调试啊?
关键是你的loadBgPic是什么时候调用的恶，如果在jpanel3初始化之前调用，那肯定是可以的，如果在jpanel3初始化后调用，第一次肯定是不会出来的，而是默认的颜色，类似于灰色。
建议把源码都贴出来看看

在paintComponent的第一行加上super.paintComponent(g);

加不加super.paintComponent(g);
结果都一样

to diggywang(Miner Lover！) 
loadBgPic是我点击按钮时候，选择背景图片文件，然后调用
所以肯定是jpanel3初始化以后
但是，现在我调用后，jPanel3.validate();
     jPanel3.repaint();
我也看到输出了，的确调用了jPanel3的paintComponent方法
但问题是，随后，jPanel3有被frame1的背景色覆盖了
如果拖动程序，让jPanel3部分被窗口遮挡，然后再移动，系统会调用paintComponent将被遮挡部分绘制上我所设置的图片。

你把JPanel的背景涂成红色，看看只是图片没出来，还是整个JPanel没出来

图片给frame的背景色挡住了

lz能不能按我说的做一下，或者把整个代码贴出来
我们在教你怎么调试呢

panel.setUIUpdate(true);

package gktsys.frame;

import java.awt.BorderLayout;

import javax.swing.JFrame;
import javax.swing.JButton;
import javax.swing.JPanel;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.Dimension;
import java.awt.event.MouseEvent;
import java.awt.event.*;
import javax.swing.BorderFactory;
import java.awt.FlowLayout;
import java.awt.Color;
import java.awt.SystemColor;
import javax.swing.JLabel;
import java.awt.Point;
import java.io.*;
import java.awt.Component;
import java.awt.Frame;
import java.awt.FileDialog;
import java.awt.Image;
import java.awt.Graphics;
import javax.swing.ImageIcon;
import java.awt.Font;

public class Frame1 extends JFrame implements ActionListener {
    //背景图片
      private ImageIcon imageIcon = null;
     transient   Image image = null;
    JPanel jPanel1 = new JPanel();
    JButton jButton2 = new JButton();
    FlowLayout flowLayout2 = new FlowLayout();
    JPanel jPanel2 = new JPanel();
    //主编辑panel
    JPanel jPanel3 = new JPanel(){
       //绘制背景
        public void paintComponent(Graphics g) {

        if(image!=null){
            g.drawImage(image,0,0,this.getWidth(),this.getHeight(),null);
        }else{
            g.setColor(new Color(100, 155, 100));
            g.fillRect(0, 0, this.getWidth(), this.getHeight());
            g.setColor(new Color(80, 135, 80));
            g.setFont(new Font("黑体", 1, 25));
            g.drawString("无背景图片", this.getWidth()/2, 30);
        }
        }
    };
    int x=0;
    int y=0;
    JButton jButton3 = new JButton();
    int START=50;
    BorderLayout borderLayout1 = new BorderLayout();
    //重载
    public void actionPerformed(ActionEvent e)
    {
        if(e.getSource().getClass().getName().equals("javax.swing.JButton"))//getSource判断哪个按钮被按下了。
        {
           //用来选择图片的
           FileDialog filedlg=new FileDialog(this,"选择背景图片");

           filedlg.setVisible(true);
           String picpath=filedlg.getDirectory();
           String  picfile=filedlg.getFile();
           System.out.println(picpath+picfile);
           imageIcon=new ImageIcon(picpath+picfile);
           image=imageIcon.getImage();
           image = image.getScaledInstance(jPanel3.getWidth(), jPanel3.getHeight(), Image.SCALE_SMOOTH);
           jPanel3.validate();
           jPanel3.updateUI();
                
        }
    }

    public Frame1() {
        try {
            jbInit();
        } catch (Exception exception) {
            exception.printStackTrace();
        }
    }
    private void jbInit() throws Exception {
        getContentPane().setLayout(borderLayout1);
        jButton2.setText("添加标签");
        jButton2.addMouseListener(new Frame1_jButton2_mouseAdapter(this));
        jButton2.addActionListener(new Frame1_jButton2_actionAdapter(this));
        jPanel1.setBorder(null);
        jPanel1.setMaximumSize(new Dimension(7670, 7670));
        jPanel1.setLayout(flowLayout2);
        this.getContentPane().setBackground(SystemColor.controlText);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setTitle("test");
        this.setVisible(true);
        jPanel2.setBorder(null);
        jPanel2.setPreferredSize(new Dimension(100, 10));
        jPanel3.setBounds(0,0,800,600);
        jPanel3.setBackground(SystemColor.activeCaptionBorder);
        jPanel3.setBorder(BorderFactory.createLoweredBevelBorder());
        jPanel3.setOpaque(false);
        jPanel3.setInputVerifier(null);
        jPanel3.setLayout(null);
        jPanel3.addMouseListener(new Frame1_jPanel3_mouseAdapter(this));
        jPanel3.addMouseMotionListener(new Frame1_jPanel3_mouseMotionAdapter(this));
        jButton3.setText("加载图像");
        jButton3.addActionListener(this);
        jPanel1.add(jButton3);
        jPanel1.add(jButton2, null);
        this.getContentPane().add(jPanel2, java.awt.BorderLayout.EAST);
        this.getContentPane().add(jPanel3, java.awt.BorderLayout.CENTER);
        this.getContentPane().add(jPanel1, java.awt.BorderLayout.NORTH);
    }

    public static void main(String[] args) {
        Frame1 frame1 = new Frame1();
        frame1.setSize(800,600);
        frame1.setVisible(true);
    }

    public void jButton2_actionPerformed(ActionEvent e) {

    }

//增加测点
    public void jButton2_mouseClicked(MouseEvent e) {

        System.out.println("clicked");
        Point p=new Point(50+START,100);
         JLabel jcedian = new JLabel();
             jcedian.setText("xin cedian");
             jcedian.setBounds(50+START,100,48,15);
              jPanel3.add(jcedian);
               START+=50;
             jPanel3.validate();
              jPanel3.repaint();
    }
}


class Frame1_jPanel3_mouseAdapter extends MouseAdapter {
    private Frame1 adaptee;
    Frame1_jPanel3_mouseAdapter(Frame1 adaptee) {
        this.adaptee = adaptee;
    }
}

class Frame1_jPanel3_mouseMotionAdapter extends MouseMotionAdapter {
    private Frame1 adaptee;
    Frame1_jPanel3_mouseMotionAdapter(Frame1 adaptee) {
        this.adaptee = adaptee;
    }
}

class Frame1_jButton2_actionAdapter implements ActionListener {
    private Frame1 adaptee;
    Frame1_jButton2_actionAdapter(Frame1 adaptee) {
        this.adaptee = adaptee;
    }

    public void actionPerformed(ActionEvent e) {
        adaptee.jButton2_actionPerformed(e);
    }
}


class Frame1_jButton2_mouseAdapter extends MouseAdapter {
    private Frame1 adaptee;
    Frame1_jButton2_mouseAdapter(Frame1 adaptee) {
        this.adaptee = adaptee;
    }

    public void mouseClicked(MouseEvent e) {
        adaptee.jButton2_mouseClicked(e);
    }
}


这是源程序，运行后，加载一jpg格式的图片，会出现黑色背景将jPanel3覆盖

黑色背景，会不会是imageBuffer没装载图片阿。

哦  没用imageBuffer啊，反正用到哪个放的图片，调试一下看图片放进去没，否则应该不会出黑色背景的。