function initPage() {
     var inputs = document.getElementsByTagName("input");
     for (var i=0; i<inputs.length; i++)  {
          if (inputs[i].className.indexOf("bt-") != -1 ) {
               inputs[i].onmouseover = function()
               {
                  this.src = this.src.replace(".gif", "-hover.gif");
               }
               inputs[i].onmouseout = function()
               {
                    this.src = this.src.replace("-hover.gif", ".gif");
               }
          }
     }
}

if (window.addEventListener)
     window.addEventListener("load", initPage, false);
else if (window.attachEvent)
     window.attachEvent("onload", initPage);