var wall = new Array();

wall[0] = "style/img/common/back_01.jpg";
wall[1] = "style/img/common/back_02.jpg";
wall[2] = "style/img/common/back_03.jpg";
wall[3] = "style/img/common/back_04.jpg";
wall[4] = "style/img/common/back_05.jpg";
wall[5] = "style/img/common/back_06.jpg";
wall[6] = "style/img/common/back_07.jpg";

var rnd = Math.floor((Math.random() * 100)) % wall.length;
document.write('<style type="text/css">body { background:#000000 url(' , wall[rnd] , ') no-repeat center top;}</style>');