How to display image at center of screen in java

package com.bornander.imagetest;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Container;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JMenuBar;
import javax.swing.JPanel;

public class MainWindow extends JFrame {
	
	private JMenuBar menuBar = new JMenuBar();
	private JPanel mainPanel = new JPanel();
	
	public MainWindow(String filename) throws IOException {
		Container contentPane = getContentPane();
		contentPane.setLayout(new BorderLayout());
		contentPane.add(menuBar, BorderLayout.NORTH);
		contentPane.add(mainPanel, BorderLayout.CENTER);
		
		menuBar.setBackground(Color.RED);
		menuBar.add(new JButton("Foo"));
		mainPanel.setBackground(Color.GREEN);
		mainPanel.setLayout(new BorderLayout());
		
		JLabel picLabel = new JLabel(new ImageIcon(ImageIO.read(new File(filename))));
		mainPanel.add(picLabel, BorderLayout.CENTER);
		
		setSize(320, 240);
		setVisible(true);
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
	}
}