buttonImg=new Array();

for (i=0; i<=7; i++){
 buttonImg[i]=new Image();
 buttonImg[i].src="images/contents_on_"+i+".gif";
}

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

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