function activateRollovers() {
  var rollovers = document.getElementsByClassName('rollover');
  for (var i=0; i < rollovers.length; i++) {
    cur_src = rollovers[i].src;
    rollovers[i].off_src = cur_src;
    rollovers[i].over_src = rollovers[i].src.match(/(.*)-over\.(jpg|gif|png)$/i) ? cur_src : cur_src.replace(/(.*)\.(jpg|gif|png)$/i, "$1-over.$2");
    rollovers[i].onmouseover = function() { this.src = this.over_src; };
    rollovers[i].onmouseout = function() { this.src = this.off_src; };
  }
}

Event.observe(window, 'load', activateRollovers);