function mkajax(){
	var majax = false;
	try {
		majax = new ActiveXObject("msxml2.XMLHTTP");
	} catch(e){
		try{
			majax=new ActiveXObject("Microsoft.XMLHTTP");
		} catch(E){
			majax=false;
		}
	}
	
	if(!majax && typeof XMLHttpRequest != 'undefined'){
		majax=new XMLHttpRequest();
	}
	
	return majax;
}


function rotaImagen(){
	var nocache=Math.round(Math.random() * 100000)
	var makeajax=mkajax();
	makeajax.open("GET","random_portafolio.php?no-cache="+nocache,true);
	makeajax.send('');
	makeajax.onreadystatechange=function(){
		if(makeajax.readyState == 4){
			document.getElementById('divPortafolio').innerHTML = makeajax.responseText;
		}
	}
	
}
<!--rotaImagen();setInterval("rotaImagen()",10000);-->
setInterval("rotaImagen()",15000);