var randombgs=["_img/background/back1.jpg", "_img/background/back2.jpg", "_img/background/back3.jpg"]

onload = function() {
	rand = Math.floor(Math.random()*randombgs.length);
	imagen = randombgs[rand];
//	alert(imagen);
	document.body.style.background = "url("+imagen+") no-repeat fixed center center";
//	alert(Math.floor(Math.random()*randombgs.length);
//	document.body.style.background='url('+randombgs[(Math.floor(Math.random()*randombgs.length))]+')');
}