var img = new Array(
'http://www.tiptop-japan.co.jp/image/home_header/header_s01.jpg',
'http://www.tiptop-japan.co.jp/image/home_header/header_s02.jpg',
'http://www.tiptop-japan.co.jp/image/home_header/header_s03.jpg',
'http://www.tiptop-japan.co.jp/image/home_header/header_s04.jpg'
);
var rnd = parseInt(Math.random() * img.length);
document.getElementById('test').style.backgroundImage = 'url(' + img[rnd] + ')';