function rollOn(element) {
  console.log(eval(imageCache[element.id + '_ro']));
  if (eval(imageCache[element.id + '_ro'])) {
   element.src = eval(imageCache[element.id + '_ro']).src;
  }
}

function rollOff(element) {
  if (eval(imageCache[element.id])) {
   element.src = eval(imageCache[element.id]).src;
  }
}