ImageIcon (Java 2 Platform SE v1.4.2)

setImageObserver

public void setImageObserver(ImageObserver observer)
Sets the image observer for the image. Set this property if the ImageIcon contains an animated GIF, so the observer is notified to update its display. For example: 
     icon = new ImageIcon(...)
     button.setIcon(icon);
     icon.setImageObserver(button);
Parameters:
observer - the image observer