ranimg = new Array(); 
ranimg[0]="img/top/catchcopy01.jpg"; 
ranimg[1]="img/top/catchcopy02.jpg"; 
ranimg[2]="img/top/catchcopy03.jpg"; 
ranimg[3]="img/top/catchcopy04.jpg"; 
xx = Math.floor(ranimg.length*Math.random()); 
ranimg = ranimg[Math.floor(xx)]; 
document.write ('<style type="text/css"><!--body div.mainbox{background: url('+ ranimg +') no-repeat left top;}--></style>');