nbuts = 10; // количество кнопок

imgon = new Array();
imgof = new Array();

for(i=0; i<nbuts; i++)
{
 imgon[i] = new Image();
 imgon[i].src = "1_"+i+".gif";
 imgof[i] = new Image();
 imgof[i].src = "0_"+i+".gif";
}

function ion (i) { (document.all)? document.images[i].src = imgon[i].src : document.nav.document.images[i].src = imgon[i].src; }
function iof (i) { (document.all)? document.images[i].src = imgof[i].src : document.nav.document.images[i].src = imgof[i].src; }