if (document.images)
{
  preload_image_object = new Image();

  image_url = new Array();
  image_url[0] = "images/btnOnasHover.png";
  image_url[1] = "images/btnOfertaHover.png";
  image_url[2] = "images/btnUslugiHover.png";
  image_url[3] = "images/btnWspolpracaHover.png";
  image_url[4] = "images/btnKontaktHover.png";

	  for (i = 0; i <= 3; i++) {
	   		preload_image_object.src = image_url[i];
	  }
}