为何设置JFrame背景失败

[younglee2013]

代码如下：
import java.awt.*;
import javax.swing.*;
public class mFramewithPictureAsBack {
public mFramewithPictureAsBack() {}
public static void main(String[] args) {
JFrame frame = new JFrame("背景图设置");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
ImageIcon img = new ImageIcon("images//a4.png");
JLabel label = new JLabel(img);
label.setBounds(0, 0, img.getIconWidth(), img.getIconHeight());
frame.getLayeredPane().add(label, new Integer(Integer.MIN_VALUE));
Container con = frame.getContentPane();
con.setLayout(new GridLayout(1, 1));
JButton btn = new JButton("按钮");
con.add(btn);
((JPanel)con).setOpaque(false);
frame.setSize(500, 300);
frame.setVisible(true);
}
}

[thisisnew]

ImageIcon img = new ImageIcon("images//a4.png"); 估计应该是这里的路径出现问题，你debug下，看看图片有没有加载进来