if (document.images) {
  img_on =new Image();  img_on.src ="fileadmin/template/img/fb_logo_on.jpg"; 
  img_off=new Image();  img_off.src="fileadmin/template/img/fb_logo_off.jpg"; 
}

function handleOver() { 
  if (document.images) document.fb_logo.src=img_on.src;
}

function handleOut() {
  if (document.images) document.fb_logo.src=img_off.src;
}