To display an image

As suggested in the article Concurrency in Swing, your button handler's query may be blocking the event dispatch thread. Using javax.swing.SwingWorker is one approach to loading images in the background, while displaying progress and keeping the GUI thread alive.

Addendum: Here's a sscce that loads the SO logo; it's been updated to handle exceptions and resize the enclosing container to fit the loaded image:

import java.awt.*;
import java.io.IOException;
import java.net.URL;
import java.util.concurrent.ExecutionException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.imageio.ImageIO;
import javax.swing.*;

/**
 * @see http://stackoverflow.com/questions/4530659
 */
public final class WorkerTest extends JFrame {

    private final JLabel label = new JLabel("Loading...");

    public WorkerTest() {
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        label.setHorizontalTextPosition(JLabel.CENTER);
        label.setVerticalTextPosition(JLabel.BOTTOM);
        this.add(label);
        this.pack();
        this.setLocationRelativeTo(null);
    }

    private void start() {
        new ImageWorker().execute();
    }

    public static void main(String args[]) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                WorkerTest wt = new WorkerTest();
                wt.setVisible(true);
                wt.start();
            }
        });
    }

    class ImageWorker extends SwingWorker<Image, Void> {

        private static final String TEST
            = "http://cdn.sstatic.net/stackexchange/img/logos/so/so-logo.png";

        @Override
        protected Image doInBackground() throws IOException {
            Image image = ImageIO.read(new URL(TEST));
            return image.getScaledInstance(640, -1, Image.SCALE_SMOOTH);
        }

        @Override
        protected void done() {
            try {
                ImageIcon icon = new ImageIcon(get());
                label.setIcon(icon);
                label.setText("Done");
                WorkerTest.this.pack();
                WorkerTest.this.setLocationRelativeTo(null);
            } catch (InterruptedException | ExecutionException e) {
                e.printStackTrace();
            }
        }
    }
}

public class SSBTest extends javax.swing.JFrame {

    /** Creates new form worker1 */
    public SSBTest() {
        initComponents();

    }

    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {

        jLabel1 = new javax.swing.JLabel();
        jPanel1 = new javax.swing.JPanel();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        jLabel1.setText("jLabel1");

        javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
        jPanel1.setLayout(jPanel1Layout);
        jPanel1Layout.setHorizontalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING).addGap(0, 348, Short.MAX_VALUE));
        jPanel1Layout.setVerticalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING).addGap(0, 210, Short.MAX_VALUE));

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING).addGroup(layout.createSequentialGroup().addContainerGap(196, Short.MAX_VALUE).addComponent(jLabel1, javax.swing.GroupLayout.PREFERRED_SIZE, 182, javax.swing.GroupLayout.PREFERRED_SIZE).addGap(178, 178, 178)).addGroup(layout.createSequentialGroup().addGap(86, 86, 86).addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE).addContainerGap(122, Short.MAX_VALUE)));
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING).addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup().addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE).addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 40, Short.MAX_VALUE).addComponent(jLabel1).addGap(36, 36, 36)));

        pack();
    }// </editor-fold>

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {

            public void run() {
             final SSBTest ssbTest = new SSBTest();
             ssbTest.setVisible(true);
             ssbTest.execute();
            }
        });
    }

 private void execute() {
  (new MeaningOfLifeFinder(jLabel1, jPanel1)).execute();
 }

 // Variables declaration - do not modify
    private javax.swing.JLabel jLabel1;
    private javax.swing.JPanel jPanel1;
    // End of variables declaration
}

class MeaningOfLifeFinder extends SwingWorker<Icon, Object> {

    JLabel label = null;
    JPanel panel;

    MeaningOfLifeFinder(JLabel label, JPanel jpanel) {
        this.label = label;
        this.panel = jpanel;
    }

    protected Icon doInBackground() throws IOException {
        URL imageurl;
        Image image = null;
        System.out.println("image loading");
        imageurl = new URL("http://maps.google.com/maps/api/staticmap"
            + "?zoom=14&size=512x512&maptype=roadmap"
            + "&markers=color:green|label:21|-15.0,-150.0&sensor=false");
        //image = (Toolkit.getDefaultToolkit().createImage(imageurl));
        image = ImageIO.read(imageurl);
     ImageIcon icon = new ImageIcon(image);
        System.out.println("image loaded...");
        return icon;

    }

    @Override
    protected void done() {
        try {
            System.out.println("image adding to label...");
            label.setIcon(get());
            //panel.add(label);
            System.out.println("image loaded to label...");
        } catch (Exception ignore) {
        }
    }
    // System.out.println(panel.getSize().width);
}