window.onload = rollover;

function rollover()
{
  if(!document.getElementById && document.getElementsByTagName) return false;

  if (document.getElementById("mouseovers"))
  {
    var source = document.getElementById("mouseovers");
    var images = source.getElementsByTagName("img");

    for (var i=0; i < images.length; i++)
    {
      var suffix = images[i].src.substring(images[i].src.lastIndexOf('.'));
      var prefix = images[i].src.slice(0,images[i].src.length - suffix.length);
      imgload = new Image();
      imgload.src = prefix+"_on"+suffix;
    }

    for (var i=0; i < images.length; i++)
    {
      images[i].onmouseover = function()
      {
        var suffix = this.src.substring(this.src.lastIndexOf('.'));   
        var prefix = this.src.slice(0,this.src.length - suffix.length);   
        this.src = prefix+"_on"+suffix;   
      }
               
      images[i].onmouseout = function()
      {
        this.src = this.src.replace("_on", '');
      }
    }
  }
}