Show 500px photo url.

500px.user.js

// ==UserScript==
// @match http://500px.com/photo/*
// ==/Userscript==

'use strict';

(function(){
    var imgPlaceHolder = document.querySelector('.nude_placeholder');
    var imgUrl = imgPlaceHolder.nextSibling.src;

    var descArea = document.querySelector('.description_text_wrap');
    var link = document.createElement('a');
    link.href = imgUrl;
    link.textContent = imgUrl;

    var style = link.style;
    style.display = 'block';
    style.fontSize = '16px';
    style.margin = '20px 0px';
    style.color = 'red';

    descArea.appendChild(link);
})();