myGifCnt = 18;
myGifTbl = new Array(
   "img/navi/01.jpg",
   "img/navi/02.jpg",
   "img/navi/03.jpg",
   "img/navi/04.jpg",
   "img/navi/05.jpg",
   "img/navi/06.jpg",
   "img/navi/07.jpg",
   "img/navi/08.jpg",
   "img/navi/09.jpg",
   "img/navi/10.jpg",
   "img/navi/11.jpg",
   "img/navi/12.jpg",
   "img/navi/13.jpg",
   "img/navi/14.jpg",
   "img/navi/15.jpg",
   "img/navi/16.jpg",
   "img/navi/17.jpg",
   "img/navi/18.jpg"
);
mySrcTbl = new Array( myGifCnt );
for (i=0; i<myGifCnt; i++){
  mySrcTbl[i] = new Image();
  mySrcTbl[i].src = myGifTbl[i];
}

function myChange( myBtnNo , myTblNo ){
  document.images["myBtn"+myBtnNo].src = mySrcTbl[myTblNo].src;
}