code: public void setImageUrl(final String imageUrl) {
  		ButtonModel model = getOrCreateComponentModel();
  		model.imageUrl = imageUrl;
  		model.image = null;
  	}
docs: Sets the URL of the image to display on the button. This is the optimum way to set an image on a button as it
  allows the image to be served from a CDN and cached on the client for future use. In this way the same "image"
  may be used for many buttons (or other components) with no extra bandwidth use. The text alternative for the
  image is generated from the button text.
  
  @param imageUrl the image url, or null for no image