RedirectTumblrImages.user.js

// ==UserScript==
// @name         Redirect Tumblr Images
// @description  Redirects to full size Tumblr images
// @version      1.0.3
// @author       heavyLobster2
// @namespace    github.com/heavyLobster2
// @downloadURL  https://gist.github.com/heavyLobster2/fb062fac9d3290330154/raw/RedirectTumblrImages.user.js
// @match        *://*.media.tumblr.com/*
// @run-at       document-start
// @grant        none
// ==/UserScript==
(function () {
    "use strict";
    
    var extensions = new Set(["jpg", "png"]);
    var sizes = new Set(["640", "540", "500", "400", "250", "100"]);
    
    var urlPieces = window.location.href.split("_");
    if (urlPieces.length < 2) return;
    
    var sizeAndExt = urlPieces[urlPieces.length - 1];
    var sizeAndExtPieces = sizeAndExt.split(".");
    if (sizeAndExtPieces.length !== 2) return;
    
    var size = sizeAndExtPieces[0];
    var extension = sizeAndExtPieces[1];
    
    if (extensions.has(extension) && sizes.has(size)) {
        var urlPrefix = "";
        for (var i = 0; i < urlPieces.length - 1; i++) {
            urlPrefix = urlPrefix + (urlPrefix === "" ? "" : "_") + urlPieces[i];
        }
        window.location.replace(urlPrefix + "_1280." + extension);
    }
})();