xtwitter_quicksave_1-0.user.js

// ==UserScript==
// @name          X/Twitter Quicksave
// @version       1.0
// @description   Double-click on expanded images to save them with a useful filename
// @author        Anonymous
// @require       https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js
// @match         https://twitter.com/*
// @exclude-match https://twitter.com/settings/*
// @grant         GM_download
// @run-at        document-end
// ==/UserScript==

(function() {
  function construct_quicksave()
  {
    var expandimgs = $("[role='dialog'] [aria-modal='true'][role='dialog'] [aria-label='Image'] img[alt]:not(.quicksave)");
    if(expandimgs.length)
    {
      $(expandimgs).each(function() {
        var iurl = this.src;

        this.addEventListener("dblclick", function(e) {
          let turl = location.href;
          let idata = iurl.match(/https?:\/\/pbs\.twimg\.com\/media\/([A-Za-z0-9_\-]+)\?/);
          let tdata = turl.match(/https?:\/\/twitter\.com\/([A-Za-z0-9_]+)\/status\/([0-9]+)\/photo\/([0-9])/);
          let srcname = iurl.replace(/format=(jpg|webp)/, "format=png");
          let newname = "twitter@" + tdata[1] + "#" + tdata[2] + "-" + tdata[3] + "_[" + idata[1] + "].png";
          GM_download({ "url": srcname, "name": newname });
        });

        this.classList.add("quicksave");
      });
    }
  }

  window.onload = function() {
    var bdy = document.querySelector("body");
    var cfg = { childList: true, subtree: true };
    var obs = new MutationObserver(function(mutations) {
      mutations.forEach(function(mutation) {
        construct_quicksave();
      });
    });
    obs.observe(bdy, cfg);
    construct_quicksave();
  };
})();