if (document.images) {
  printof = new Image();
  printof.src = "../img/printof.gif";
  printon = new Image();
  printon.src = "../img/printon.gif";
} 
     
function onKnopf(imgName) {
  if (document.images)
  document[imgName].src = eval(imgName + 'on.src')
} 
     
function offKnopf(imgName) {
  if (document.images)
  document[imgName].src = eval(imgName + 'of.src')
}