public class Iconworld extends JFrame {
  private Iconworld() {
    pack();
    setTitle("массивы ё");
    setSize(300,300);
    setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    setResizable(false);
    setLocationRelativeTo(this);
    setVisible(true);
    ImageIcon icon = new ImageIcon(getClass().getResource("0.png"));
    //ImageIcon icon = new ImageIcon("res/0.png");
    setIconImage(icon.getImage());
  }

  public static void main(String[] args) {
    new Iconworld();
  }
}

ImageIcon icon = new ImageIcon("res/0.png");