bgc = new Array();
bgc[0] = "/images/main_image01.jpg";
bgc[1] = "/images/main_image02.jpg";
bgc[2] = "/images/main_image03.jpg";
bgc[3] = "/images/main_image04.jpg";
n = Math.floor(Math.random()*bgc.length);
document.write('<img src="'+bgc[n]+'" width="800" height="140" />');