// cross-browser event handling      
      function addEvent(element, evType, fn, useCapture) {
        if (element.addEventListener) {
          element.addEventListener(evType, fn, useCapture); 
          return true; 
        } else if (element.attachEvent) {
          var r = element.attachEvent('on' + evType, fn); 
          return r; 
        } else {
          element['on' + evType] = fn;
        }
      }

//add a listener on page load
       addEvent(window, 'load', addListeners, false);

      function addListeners() {
        var allButtons = document.getElementsByTagName("input");
        for(i=0;i< allButtons.length; ++i) {
         var button = allButtons[i];
         var button_type = button.getAttribute("type");
         if(button_type = 'image') {
          addEvent(button, 'mouseover', mouse_over, false);
          addEvent(button, 'mouseout', mouse_out, false);
         }
        }
      }


      function mouse_over() {
      if (window.event) { 
        var tg = window.event.srcElement;
      } 
      else { var tg = this;
      }
//replaces a period and any number of chars after the period(constituting the file extension) with _over plus the file extension (stored in $1)
      var s = tg.src.replace(/(\.[^.]+)$/, '_over$1');
      tg.src = s;
      }
    
        
      function mouse_out() {
      if (window.event) { 
        var tg = window.event.srcElement; 
      }
      else { var tg = this;
      }
      var s = tg.src.replace(/_over(\.[^.]+)$/, '$1');
      tg.src = s;
      }