1.创建一个类ImagePanel,代码如下

import java.awt.BorderLayout;
import javax.swing.JPanel;
import java.io.File;
import javax.imageio.ImageIO;
import java.io.IOException;
import java.awt.Image;
import java.awt.Graphics;
import java.awt.Graphics2D;

class ImagePanel extends JPanel {
    public ImagePanel() {
        try {
            File f = new File("Img/Login.gif");
            img = ImageIO.read(f);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private Image img;

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;

        if (img != null) {
            g2.drawImage(img, 0, 0, getWidth(), getHeight(),
                         0, 0, img.getWidth(null), img.getHeight(null), null);
        }
    }
} 

在新建的JPanel面板中调用:

把public class Welcome
    extends JPanel

改为public class Welcome
    extends ImagePanel

即可实现背景.

在对话框中使用如下:

public class Login
    extends JDialog {
  String pass;
  static String name;
  ImagePanel panel1 = new ImagePanel();  //使用ImagePanel类载入的背景替换默认Jpanel1做为背景