bgc = new Array();
 bgc[0] = "url(../images/bg/00.jpg)";
 bgc[1] = "url(../images/bg/01.jpg)";
 bgc[2] = "url(../images/bg/02.jpg)";
 bgc[3] = "url(../images/bg/03.jpg)";
 n = Math.floor(Math.random()*bgc.length);
 document.body.style.backgroundImage = bgc[n]