Twitter Image Helper modifies image sources so that the best available image quality is always being used in tweets. Also adds a download button for images in tweets and redirects direct image links from Twitter to show the largest available size. Also: No more jpg-orig etc. image extensions when downloading!

twitter.imagehelper.user.js

// ==UserScript==
// @name         Twitter Image Helper v2
// @namespace    Twitter Image Helper v2
// @version      0.1
// @description  Twitter Image Helper modifies image sources so that the best available image quality is always being used in tweets. Also adds a download button for images in tweets and redirects direct image links from Twitter to show the largest available size. Also: No more jpg-orig etc. image extensions when downloading!
// @author       You
// @require      http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js
// @require      https://raw.githubusercontent.com/pie6k/jquery.initialize/master/jquery.initialize.min.js
// @include      *pbs.twimg.com/media/*
// @include      *twitter.com*
// ==/UserScript==

this.$ = this.jQuery = jQuery.noConflict(true);
var currentUrl = window.location.href;
var mediaIdRegex = /(?<=\/)([a-zA-Z0-9\_\-]){15}/;
var mediaSizeRegex = /(name=)(.*$)/;
var mediaFormatRegex = /(format=)([aA-zZ]{3})/;
var linkHtml = '<a target="_blank" href="$LINK" style="font-family: Arial, Helvetica, sans-serif; z-index: 9999; position:absolute; margin: 8px; height:15px; width: $LWIDTH; text-align: center; line-height:15px; padding: 3px 10px; border-radius: 3px; left: 0; right: 0; margin-left: auto; margin-right: auto; margin-top: 8px; cursor:pointer; background-color: rgba(20,20,20,0.75); color: white; font-size: 11px; text-decoration: none;" class="download-button" download>Download ($IWx$IH)</a>';

function getMeta(url){
    var r = $.Deferred();
    $('<img/>').attr('src', url).load(function(){
        var s = {w:this.width, h:this.height};
        r.resolve(s)
    });
    return r;
}

var getOriginalSize = function(link) {
    if (!link.endsWith("orig")) {
        var mediaId = link.match(mediaIdRegex)[0];
        var mediaFormat;
        console.log("Got media Id: " + mediaId);
        if (link.indexOf("format=") == -1) {
            console.log("No media format found, assuming link ends in extension.");
            mediaFormat = link.split(".").pop().split(":")[0];
            console.log("Got media format: " + mediaFormat);
        }
        else {
            mediaFormat = link.match(mediaFormatRegex)[2];
            console.log("Got media format: " + mediaFormat);
        }
        return "https://pbs.twimg.com/media/" + mediaId + "?format=" + mediaFormat + "&name=orig";
    }
};

if (currentUrl.indexOf("pbs.twimg.com/media/") > 0){
    if (!currentUrl.endsWith("orig")) {
        window.location.href = getOriginalSize(currentUrl);
    }
}
else {
    $("img").initialize(function() {
        var imgLink = $(this).attr('src');
        if (imgLink.indexOf("pbs.twimg.com/media") != -1) {
            var parentEl = $(this).parent().parent().parent();
            var newImgLink = getOriginalSize(imgLink);
            if (parentEl.width() < 150) {
                $(parentEl).prepend(linkHtml.replace("$LINK", newImgLink).replace("($IWx$IH)", "").replace("$LWIDTH", "50px"));
            }
            else {
                getMeta(newImgLink).done(function(imgDims){
                    $(parentEl).prepend(linkHtml.replace("$LINK", newImgLink).replace("$IW", imgDims.w).replace("$IH", imgDims.h).replace("$LWIDTH", "115px"));
                });
            }
        }
    });

    $('body').on('click', '.download-button', function(event) {
        event.stopPropagation();
    });
}