var preloadFlag = false;

if (document.images)
{
  var onImgArray = new Array();
  onImgArray['home'] = new Image();
  onImgArray['aboutus'] = new Image();
  onImgArray['sbexam'] = new Image();
  onImgArray['categories'] = new Image();
  onImgArray['links'] = new Image();
  onImgArray['contact'] = new Image();
  onImgArray['home'].src = 'images/home_over.gif';
  onImgArray['aboutus'].src = 'images/aboutus_over.gif';
  onImgArray['sbexam'].src = 'images/sbexam_over.gif';
  onImgArray['categories'].src = 'images/products_over.gif';
  onImgArray['links'].src = 'images/links_over.gif';
  onImgArray['contact'].src = 'images/contact_over.gif';

  var offImgArray = new Array();
  offImgArray['home'] = new Image();
  offImgArray['aboutus'] = new Image();
  offImgArray['sbexam'] = new Image();
  offImgArray['categories'] = new Image();
  offImgArray['links'] = new Image();
  offImgArray['contact'] = new Image();
  offImgArray['home'].src = 'images/home.gif';
  offImgArray['aboutus'].src = 'images/aboutus.gif';
  offImgArray['sbexam'].src = 'images/sbexam.gif';
  offImgArray['categories'].src = 'images/products.gif';
  offImgArray['links'].src = 'images/links.gif';
  offImgArray['contact'].src = 'images/contact.gif';

  preloadFlag = true;
}

function imageOn(imgName)
{
  if (preloadFlag)
  {
    document.images[imgName].src = onImgArray[imgName].src;
  }
}

function imageOff(imgName)
{
  if (preloadFlag)
  {
    document.images[imgName].src = offImgArray[imgName].src;
  }
}