var image1 = new Image();
image1.src = "img/nav.gif";
var image2 = new Image();
image2.src = "img/nav_act.gif";
var image3 = new Image();
image3.src = "img/cont.gif";
var image4 = new Image();
image4.src = "img/h2.gif";


function roll(img_name1, img_src1, img_name2, img_src2)
   {
   document[img_name1].src = img_src1;
   document[img_name2].src = img_src2;
   }