https://blog.marcnuri.com/dynamic-icons-for-your-jcomponents-create-an-icon-jbutton-with-dynamic-icons/

ImageButton.java

/*
 * ImageButton.java
 *
 * Created on 30 de mayo de 2007, 8:35
 */

package blogtests;

import java.awt.image.BufferedImage;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;

/**
 *
 * @author  Marc Nuri San Félix
 */
public class ImageButton extends javax.swing.JFrame {
    
    /** Creates new form imageButton */
    public imageButton() {
        initComponents();
        test();
    }

    public void test(){
        try {
            URL pepe = new URL("http://search.yahooapis.com/ImageSearchService/V1/imageSearch?appid=YahooDemo&query=client%20icon&results=1&format=gif");
            BufferedReader br = new BufferedReader(new InputStreamReader(pepe.openStream()));
            String text ="";
            String line = null;
            while((line = br.readLine()) != null){
                text += line+"\n";
            }
            text = text.substring(text.indexOf("<Thumbnail>"));
            text = text.substring(text.indexOf("<Url>")+5, text.indexOf("</Url>"));
            BufferedImage img = ImageIO.read(new URL(text));
            jButton1.setIcon(
                    new ImageIcon(img));
        } catch (MalformedURLException ex) {
            ex.printStackTrace();
        } catch (IOException ex) {
            ex.printStackTrace();
        }

    }
    /** 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.
     */
    // <editor-fold defaultstate="collapsed" desc=" Generated Code ">//GEN-BEGIN:initComponents
    private void initComponents() {
        jButton1 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        jButton1.setText("Customers");

        org.jdesktop.layout.GroupLayout layout = new org.jdesktop.layout.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING)
            .add(layout.createSequentialGroup()
                .addContainerGap()
                .add(jButton1, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                .add(305, 305, 305))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(org.jdesktop.layout.GroupLayout.LEADING)
            .add(layout.createSequentialGroup()
                .addContainerGap()
                .add(jButton1, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, org.jdesktop.layout.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                .addContainerGap())
        );
        pack();
    }// </editor-fold>//GEN-END:initComponents
    
    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new imageButton().setVisible(true);
            }
        });
    }
    
    // Variables declaration - do not modify//GEN-BEGIN:variables
    private javax.swing.JButton jButton1;
    // End of variables declaration//GEN-END:variables
    
}

Source code for very old example in https://blog.marcnuri.com/dynamic-icons-for-your-jcomponents-create-an-icon-jbutton-with-dynamic-icons/