$(document).ready(function(){

  $(".jHover").hover(
   function() {
    curr = $(this).find("img").attr("src");
    overlen = curr.length;
    over = curr.substr(0, overlen-4);
    over = over+'_on.png';
    $(this).find("img").attr({ src: over});
   },
   function() {
    $(this).find("img").attr({ src: curr});
   }
  )
  
  $(".jHover").find("img").each(function(i) {
    temp = this.src;
    prelen = temp.length;
    pre = temp.substr(0, prelen-4);
    pre = pre+'_on.png';
    preload_image_object = new Image();
    preload_image_object.src = pre;
  });

});