if (document.images) {

var image1 = new Image();
image1.src = "images/nav-home-on.png";
var image2 = new Image();
image2.src = "images/nav-home-off.png";
var image3 = new Image();
image3.src = "images/nav-aboutus-on.png";
var image4 = new Image();
image4.src = "images/nav-aboutus-off.png";
var image5 = new Image();
image5.src = "images/nav-mushroom101-on.png";
var image6 = new Image();
image6.src = "images/nav-mushroom101-off.png";
var image7 = new Image();
image7.src = "images/nav-products-on.png";
var image8 = new Image();
image8.src = "images/nav-products-off.png";
var image9 = new Image();
image9.src = "images/nav-research-on.png";
var image10 = new Image();
image10.src = "images/nav-research-off.png";
var image11 = new Image();
image11.src = "images/nav-news-on.png";
var image12 = new Image();
image12.src = "images/nav-news-off.png";
var image13 = new Image();
image13.src = "images/nav-wheretobuy-on.png";
var image14 = new Image();
image14.src = "images/nav-wheretobuy-off.png";
var image15 = new Image();
image15.src = "images/nav-contact-on.png";
var image16 = new Image();
image16.src = "images/nav-contact-off.png";
var image17 = new Image();

}

function roll(img_name, img_src)
   {
   document[img_name].src = img_src;
   }