var wall = new Array();

/* 使用する背景画像を好きなだけ記述 */
wall[0] = "img/back1.jpg";
wall[1] = "img/back2.jpg";
wall[2] = "img/back3.jpg";
wall[3] = "img/back4.jpg";
wall[4] = "img/back5.jpg";

var rnd = Math.floor((Math.random() * 100)) % wall.length;
document.write('<style type="text/css">#main { background-image:url(' , wall[rnd] , '); background-repeat:no-repeat; }</style>');