wall = new Array();

/* 使用する背景画像を好きなだけ記述 */
wall[0] = "./image/index/main_image1.jpg";
wall[1] = "./image/index/main_image2.jpg";
wall[2] = "./image/index/main_image3.jpg";
wall[3] = "./image/index/main_image4.jpg";
wall[4] = "./image/index/main_image5.jpg";

rnd = Math.round(Math.random() * (wall.length - 1));
document.write('<style type="text/css">.MainImg { background-image:url(' , wall[rnd] , '); }</style>');