if(document.images) {
   img_email_a=new Image();
   img_email_a.src="../images/address/email_a.gif";
   img_email_na=new Image();
   img_email_na.src="../images/address/email_na.gif";
} 

function on_email() {
   if(document.images) document.email.src=img_email_a.src;
}

function off_email() {
   if(document.images) document.email.src=img_email_na.src;
}