buttonImg=new Array();

for(i=0;i<=15;i++){
            buttonImg[i] = new Image();
            buttonImg[i].src="../img/b_" + i + "_on.jpg";}
 
 function imageOn(n)
 {document.images['menu'+n].src=buttonImg[n].src}

function imageOff(n)
 {document.images['menu'+n].src="../img/b_" + n + "_off.jpg";}