load = 0;
var bt = new Array(1,2,3);
var onbt   = new Array();
var offbt  = new Array();

for(i=0; i < bt.length; i++){
   onbt[i]   = new Image();
   offbt[i]  = new Image();
   onbt[i].src   = "img/bt_dc_0" + bt[i] + "_b.gif";
   offbt[i].src  = "img/bt_dc_0" + bt[i] + "_a.gif";
}
function ChgBt(num,flag){
   if(load){
      document.images['BT' + num].src = flag ? onbt[num].src : offbt[num].src;
   }
}