var c = 1;

/* Preloading images */
var image1 = new Image();
image1.src = "images/banner1.png";
var image2 = new Image();
image2.src = "images/banner2.png";
var image3 = new Image();
image3.src = "images/banner3.png";
var image4 = new Image();
image4.src = "images/banner4.png";

function disp_img(w)
   {
   if (c == 5)
      {
      c = 1;
      }
   var img_src = "images/banner" + c + ".png";
   document.ani.src = img_src;
   c++;
   }
t = setInterval("disp_img(c)", 2500);