function preload(){
buttonImg = new Array();
for (i=0; i<=9; i++){
buttonImg[i] = new Image();
buttonImg[i].src = "img/btn_" + i + "_on.gif"
}
}

function imageOn(n){
document.images['menu'+n].src = buttonImg[n].src
}

function imageOff(n){
document.images['menu'+n].src = "img/btn_" + n + "_of.gif";
}