var n = 4;
var f = false;
var b = new Array;

function prel(){
	if(document.images) {
		for(i=1;i<=n;i++){
		  str = 'buttons/b' + i + '_o.jpg';
		  b[i-1] = new Image();
		  b[i-1].src = str;
		  }
		f = true;
 	}
}

function on(im){
	if (document.images && (f == true)) {document.images[im].src="buttons/"+im+"_o.jpg";}
}

function off(im){
	if (document.images && (f == true)) {document.images[im].src = "buttons/"+im+".jpg";}
}