Loading resources like images while running project distributed as JAR archive

First of all, change this line :

image = ImageIO.read(getClass().getClassLoader().getResource("resources/icon.gif"));

to this :

image = ImageIO.read(getClass().getResource("/resources/icon.gif"));

More info, on as to where lies the difference between the two approaches, can be found on this thread - Different ways of loading a Resource

For Eclipse:

• How to add Images to your Resource Folder in the Project

For NetBeans:

• Handling Images in a Java GUI Application
• How to add Images to the Project

For IntelliJ IDEA:

• Right-Click the src Folder of the Project. Select New -> Package
• Under New Package Dialog, type name of the package, say resources. Click OK
• Right Click resources package. Select New -> Package
• Under New Package Dialog, type name of the package, say images. Click OK
• Now select the image that you want to add to the project, copy it. Right click resources.images package, inside the IDE, and select Paste

• Use the last link to check how to access this file now in Java code. Though for this example, one would be using

  getClass().getResource("/resources/images/myImage.imageExtension");

• Press Shift + F10, to make and run the project. The resources and images folders, will be created automatically inside the out folder.

If you are doing it manually :

• How to add Images to your Project
• How to Use Icons
• A Little extra clarification, as given in this answer's first code example.

QUICK REFERENCE CODE EXAMPLE(though for more detail consider, A little extra clarification link):

package swingtest;

import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.*;

/**
 * Created with IntelliJ IDEA.
 * User: Gagandeep Bali
 * Date: 7/1/14
 * Time: 9:44 AM
 * To change this template use File | Settings | File Templates.
 */
public class ImageExample {

    private MyPanel contentPane;

    private void displayGUI() {
        JFrame frame = new JFrame("Image Example");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        contentPane = new MyPanel();

        frame.setContentPane(contentPane);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    private class MyPanel extends JPanel {

        private BufferedImage image;

        public MyPanel() {
            try {
                image = ImageIO.read(MyPanel.class.getResource("/resources/images/planetbackground.jpg"));
            } catch (IOException ioe) {
                ioe.printStackTrace();
            }
        }

        @Override
        public Dimension getPreferredSize() {
            return image == null ? new Dimension(400, 300): new Dimension(image.getWidth(), image.getHeight());
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.drawImage(image, 0, 0, this);
        }
    }

    public static void main(String[] args) {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                new ImageExample().displayGUI();
            }
        };
        EventQueue.invokeLater(runnable);
    }
}

There's a much easier way to load and set an image as a frame icon:

frame.setIconImage(
    new ImageIcon(getClass().getResource("/resources/icon.gif")).getImage());

And thats all :)! You don't even have to use a try-catch block because ImageIcon does not throw any declared exceptions. And due to getClass().getResource(), it works both from file system and from a jar depending how you run your application.

If you need to check whether the image is available, you can check if the URL returned by getResource() is null:

URL url = getClass().getResource("/resources/icon.gif");
if (url == null)
    System.out.println( "Could not find image!" );
else
    frame.setIconImage(new ImageIcon(url).getImage());

The image files must be in the directory resources/ in your JAR, as shown in How to Use Icons and this example for the directory named images/.