var 
 Mlist = new Array();

function CacheImage(ImageSource) {
   var ImageObject = new Image();
   ImageObject.src = ImageSource;
   return ImageObject;
}

function LoadBtmPictures(slName, OnPic, OffPic) {
  var sublist = new Array();
  sublist["On"] = CacheImage(OnPic);
  sublist["Off"] = CacheImage(OffPic);
  Mlist[slName] = sublist;
}

function SetImgByTagName(tagName, arrName, picName) {
  document[tagName].src = Mlist[arrName][picName].src;
}

function SetImgItSelf(arrName, picName){
  event.srcElement.src = Mlist[arrName][picName].src;
}

function SetImgItSelfByName(picName){
  event.srcElement.src = Mlist[event.srcElement.name][picName].src;
}