myGifCnt = 6;
myGifTbl = new Array(     // ボタン画像テーブルを作成
   "img/button_login0.jpg",
   "img/button_login1.jpg",
   "img/button_regist0.jpg",
   "img/button_regist1.jpg",
   "img/button_regist_b0.jpg",
   "img/button_regist_b1.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;
}