function MouseOver(el,which){
  el.src=el.getAttribute(which || "origsrc");
}

function MouseOverSetup(){
  var x = document.getElementsByTagName("img");
  for (var i=0;i<x.length;i++){
    var over = x[i].getAttribute("over");
    if (!over) continue;
      
    // preload image
    // comment the next two lines to disable image pre-loading
    x[i].over_img = new Image();
    x[i].over_img.src=over;
    // set event handlers
    x[i].onmouseover = new Function("MouseOver(this,'over');");
    x[i].onmouseout = new Function("MouseOver(this);");
    // save original src
    x[i].setAttribute("origsrc",x[i].src);
  }
}

var PreMouseOverOnload =(window.onload)? window.onload : function(){};
window.onload = function(){PreMouseOverOnload(); MouseOverSetup();}