window.onload = function(){
//var url = "a.jpg,b.jpg,c.jpg".split(',');
var url=[];
url[0] = 'bg_sub1.jpg';
url[1] = 'bg_sub2.jpg';
url[2] = 'bg_sub1.jpg';
var n = Math.floor(Math.random() * url.length);
var elm = document.getElementById('bg_wrap');
elm.style.backgroundImage = 'url(https://www.gakkou.co.jp/kyoto-navi.com/files/img/common/' + url[n] + ')';
}