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

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

addEvent(window, 'load', SimpleSwapSetup);

// swaped image tag should have attribute - overscr="hover-item.gif"
// Ex: <script type="text/javascript" src="simpleswap.js" /><img src="bla-bla.gif" oversrc="hover-bla-bla.gif" />