Event.observe(window, 'load', function() //automatically adds the mouseover functionality once the DOM is loaded
{ 
var hoverImages = $$('.hover');
for(var i=0, n=hoverImages.length; i<n; i++){
hoverImages[i].onmouseover=function(){
this.src = this.src.replace("_off.","_over.");
};

hoverImages[i].onmouseout=function(){
this.src = this.src.replace("_over.","_off.");
};
}

});