twimg-icon.user.js

// ==UserScript==
// @name        twimg-icon
// @namespace   github.com/mavc
// @description Download source images on Twitter.
// @include     https://twitter.com/*
// @exclude     https://twitter.com/i/*
// @version     1.1.1
// @require     https://ajax.googleapis.com/ajax/libs/jquery/3.1.0/jquery.min.js
// @grant       none
// ==/UserScript==
$(document).ready(function () {
  const SELECTOR = 'div.AdaptiveMedia-container img';

  let findImages = (tweet) => {
    return tweet
      .find(SELECTOR)
      .map((i, img) => $(img).attr('src'))
      .get()
      .filter(src => /\.twimg\./.test(src))
      .map(src => `${src}:orig`);
  };

  let makeIcon = (src) => {
    let icon = $('<span>').addClass('Icon Icon--camera');
    icon = $('<span>')
      .addClass('IconContainer')
      .append(icon);          
    icon = $('<span>')
      .addClass('ProfileTweet-actionButton u-textUserColorHover')
      .append(icon);          
    return $('<a>')
      .css({display: 'inline-block', 'min-width': '25px'})
      .addClass('twimg-button')
      .attr('href', src)
      .attr('target', '_blank')
      .append(icon);
  };

  $('body')
    .on('mouseenter', 'div.tweet', function () {
      const tweet = $(this);
      const imgs = findImages(tweet);

      if (imgs.length > 0) {
        console.debug('Images: ', imgs); 
               
        tweet.find('.twimg-button').remove();
        const footer = tweet.find('div.stream-item-footer div.ProfileTweet-actionList');
        imgs.forEach(src => { footer.append(makeIcon(src)); });
      }
    });
});