function rollon() {
  if (window.event.srcElement.className == "NavLink") {
    var imagename = window.event.srcElement.src;
	imagename = imagename.replace('.jpg' , '_over.jpg')
	window.event.srcElement.src = imagename;
  }
}

function rolloff() {
  if (window.event.srcElement.className == "NavLink") {
    var imagename = window.event.srcElement.src;
	imagename = imagename.replace('_over.jpg' , '.jpg')
	window.event.srcElement.src = imagename;
  }
}

document.onmouseover = rollon;

document.onmouseout = rolloff;