window.onload = function(){
var url = new URL(window.location.href);

var ele = document.getElementsByTagName("body")[0];
var n = Math.floor(Math.random() * 3); // 3枚の画像がある場合
ele.style.backgroundImage = "url("+url.protocol+"//"+url.hostname+"/img/titleimg/"+n+".png)";
}