E6Saver_local.user.js

// ==UserScript==
// @name         E6Saver
// @namespace    http://tampermonkey.net/
// @version      0.3a
// @description  try to take over the world!
// @author       You
// @match        https://e621.net/post/show/*
// @grant        none
// ==/UserScript==

function addJQuery(callback) {
    var script = document.createElement("script");
    script.setAttribute("src", "//ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js");
    script.addEventListener('load', function() {
        var script = document.createElement("script");
        script.textContent = "window.jQ=jQuery.noConflict(true);(" + callback.toString() + ")();";
        document.body.appendChild(script);
    }, false);
    document.body.appendChild(script);
}

function main(){
    // test server host
    var testSrv = "192.168.2.11";

    console.log("main called");
    jQ("a:contains('Download')").click(function(event){
        event.preventDefault();
        console.log(jQ(this).attr("href"));

        var encodedImgLink = encodeURI(jQ(this).attr("href").replace("https://", "//"));
        window.open("http://"+testSrv+":98/size?origin=e621&imgUrl="+ encodedImgLink, window);
    });
}

addJQuery(main);