setImageOnbtn

import java.awt.Image;
import javax.swing.ImageIcon;
import javax.swing.JButton;

public class setImageOnbtn {

    public void setImageOnLabel (String path, JButton label){
      ImageIcon imageWL = new ImageIcon(path);
      Image scaledImageWL = imageWL.getImage().getScaledInstance(label.getWidth(),label.getHeight(),label.getHeight());
      ImageIcon scaledIconWL = new ImageIcon(scaledImageWL);
      label.setIcon(imageWL);
    }

}

Ejemplo:
 setImageOnbtn  btnSalir = new setImageOnbtn();
 btnSalir.setImageOnLabel(appPath().toString()+ "/images/salir.png",cmdSalir);