var img1=new Image(50, 47, 'butt1.jpg');
var img2=new Image(50, 47, 'butt2.jpg');

function light(num){
num+=1;
var el=document.images.item(num);
el.src='butt1.jpg';
}

function fade(num){
num+=1;
var el=document.images.item(num);
el.src='butt2.jpg';
}