function markBackground(tabname,tabvalue) {
    tabname.style.cursor='pointer';
	switch(tabvalue) {
	  case "about": document.getElementById('mainmenu_about').src="images/mainmenu_about_sel.gif";break;
	  case "nt": document.getElementById('mainmenu_nt').src="images/mainmenu_nt_sel.gif";break;
	  case "fields": document.getElementById('mainmenu_fields').src="images/mainmenu_fields_sel.gif";break;
	  case "faqs": document.getElementById('mainmenu_faqs').src="images/mainmenu_faqs_sel.gif";break;
	  case "contact": document.getElementById('mainmenu_contact').src="images/mainmenu_contact_sel.gif";break;
	}
  }
  function unmarkBackground(tabname,tabvalue) {
    tabname.style.cursor='pointer';
	switch(tabvalue) {
	  case "about": document.getElementById('mainmenu_about').src="images/mainmenu_about.gif";break;
	  case "nt": document.getElementById('mainmenu_nt').src="images/mainmenu_nt.gif";break;
	  case "fields": document.getElementById('mainmenu_fields').src="images/mainmenu_fields.gif";break;
	  case "faqs": document.getElementById('mainmenu_faqs').src="images/mainmenu_faqs.gif";break;
	  case "contact": document.getElementById('mainmenu_contact').src="images/mainmenu_contact.gif";break;
	}
  }