var img = new Array(
'http://digital-book.net/image/home_header/01_header.jpg',
'http://digital-book.net/image/home_header/02_header.jpg',
'http://digital-book.net/image/home_header/03_header.jpg',
'http://digital-book.net/image/home_header/04_header.jpg',
'http://digital-book.net/image/home_header/05_header.jpg'
);
var rnd = parseInt(Math.random() * img.length);
document.getElementById('test').style.backgroundImage = 'url(' + img[rnd] + ')';