//In super class
protected Image image;

public BinaryGate(String imgUrl){
    java.net.URL url = getClass().getResource(imgUrl);
    image = new ImageIcon(url).getImage();
    this.setSize(image.getWidth(null), image.getHeight(null));
}

///....in sub class...
public ANDGate(){
    super("images/AND.gif");
}