var doc_url = document.URL;
if (document.images) {
  if (doc_url.indexOf("event_") >= 0) {
  document.images["events"].src = "/images/nav_events_on.gif";
  } else if (doc_url.indexOf("halfpricetickets") >= 0) {
  document.images["halfprice"].src = "/images/nav_halfprice_on.gif";
  } else if (doc_url.indexOf("ticketbooths") >= 0) {
  document.images["ticketbooth"].src = "/images/nav_ticketbooth_on.gif";
  } else if (doc_url.indexOf("aboutus") >= 0) {
  document.images["aboutus"].src = "/images/nav_aboutus_on.gif";
  } 
}