function roll_over(img_name, img_src)
   {
  
 var img = document.getElementById(img_name);
 img.src = img_src;
   }

function preloadImages()
{
     // counter
     var i = 0;
     // create object
     imageObj = new Image();
     // set image list
     images = new Array();
     images[0]="_img/home/nav_1_on.gif"
     images[1]="_img/home/nav_2_on.gif"
     images[2]="_img/home/nav_3_on.gif"
     images[3]="_img/home/nav_4_on.gif"
     images[4]="_img/home/nav_5_on.gif"
     images[5]="_img/home/nav_6_on.gif"
     images[6]="_img/home/nav_7_on.gif"
     images[7]="_img/home/nav_8_on.gif"
     // start preloading
     for(i = 0; i <= images.length; i++) 
     {
          imageObj.src=images[i];
          document.write("<img src='" + imageObj.src + "' style='display: none;'/>");
     }
}
preloadImages();