function menuOnMouseOver(event) {
  var element = event.element();
  var src = element.readAttribute('src');
  var hoverSrc = src.gsub('.png','-hover.png');
  element.src = hoverSrc;
}

function menuOnMouseOut(event) {
  var element = event.element();
  var hoverSrc = element.readAttribute('src');
  var src = hoverSrc.gsub('-hover','');
  element.src = src;
}

document.observe("dom:loaded", function() {
  $$('.menu-link-img').each(function(item){
    var src = item.readAttribute('src');
    var hoverSrc = src.gsub('.png','-hover.png');
    var img = new Image(100,100);
    img.src = hoverSrc;
    item.observe('mouseover', menuOnMouseOver);
    item.observe('mouseout', menuOnMouseOut);
  });
});