window.addEvent("domready", function() {

 	 var nav = $$('img.portfolio_nav_link'); 	 
 	 for (var i=0; i < nav.length; i++) {
 	  nav[i].onmouseover=function() { 	    
      this.src=this.src.replace(new RegExp(".jpg\\b"), "_o.jpg");
    };
  	nav[i].onmouseout=function() {
  	  this.src=this.src.replace(new RegExp("_o\\b"), "");
  	};
 	 } 	 
});