var size = 13; // Number of alternative images var 
images = new Array ("images/header_image1.jpg",
				    "images/header_image2.jpg",
				    "images/header_image3.jpg",
					"images/header_image4.jpg",
					"images/header_image5.jpg",
					"images/header_image6.jpg",
					"images/header_image7.jpg",
					"images/header_image8.jpg",
					"images/header_image9.jpg",
					"images/header_image10.jpg",
					"images/header_image11.jpg",
					"images/header_image12.jpg",
					"images/header_image13.jpg");

function pickRandom(range) {
	if (Math.random) return Math.round(Math.random() * (range-1)); 
	
	else { 
		var now = new Date(); return (now.getTime() / 1000) % range; 
		} 
	}
	
var choice = pickRandom(size);