var img = new Array(
'http://www.lifehopenet.com/image/header/01.png',
'http://www.lifehopenet.com/image/header/02.png',
'http://www.lifehopenet.com/image/header/03.png',
'http://www.lifehopenet.com/image/header/04.png'
);
var rnd = parseInt(Math.random() * img.length);
document.getElementById('test').style.backgroundImage = 'url(' + img[rnd] + ')';