var imageURL = "i/blank2.gif";

if (document.images) {
     var blank = new Image();
     blank.src = "i/blank2.gif";

     var sales = new Image();
     sales.src = "i/sales.gif";
}

function changeImage() {
     if (document.images) {
          if (imageURL == "i/blank2.gif") imageURL = "i/sales.gif";
          else imageURL = "i/sales.gif";

         document.myImage.src = imageURL;
     }
}