function image_swap(myID)
{
   var strOver  = "_on"
   var strOff = "_off"
   var oSpan = document.getElementById(myID)
   var currentAlphaImg = oSpan.src
   
   if (currentAlphaImg.indexOf(strOver) != -1)
      oSpan.src = currentAlphaImg.replace(strOver,strOff)
   else
      oSpan.src = currentAlphaImg.replace(strOff,strOver)
}