var wall = new Array();

wall[0] = "/center/images/header_img_01.jpg";
wall[1] = "/center/images/header_img_02.jpg";
wall[2] = "/center/images/header_img_03.jpg";

var rnd = Math.floor((Math.random() * 100)) % wall.length;
document.write('<style type="text/css">#top { background:url(' , wall[rnd] , ') no-repeat 100% 30px; }</style>');