window.onload = function(){

  var nodes = $$("img.mouseover");

  nodes.each(function(node)
  {
    Event.observe(node, 'mouseover', function(event){
      element = Event.element(event);
      element.src = element.src.replace(/(.jpg|.gif)$/, '_hover$1');
    });
    Event.observe(node, 'mouseout', function(event){
      element = Event.element(event);
      element.src = element.src.replace(/_hover/, '');
    });
  });
}