function imgSwap(oImg)
{
   var strOver  = "_on";    // image to be used with mouse over
   var strOff = "_off";     // normal image
   var strImg = oImg.src;
   if (strImg.indexOf(strOver) != -1) 
      {oImg.src = strImg.replace(strOver,strOff);}
   else
      {oImg.src = strImg.replace(strOff,strOver);}
}