function randomHeader(layername) {
	
   var randomnumber=(Math.floor(Math.random()*8)+1);

   if (document.all) document.all(layername).style.background='url(img/header_' + randomnumber + '.jpg)'; 
   else if (document.layers) document.layers[layername].background.src='img/header_'+randomnumber+'.jpg'; 

}