Copy the alt text in tweets into the title text, so that you can see it on hover.

twitter-alt-to-title.user.js

// ==UserScript==
// @name     Twitter Alt-Text to Title-Text
// @description Copy the alt attribute of twitter images into the title attribute, so that I can see the alt text on hover.
// @version  1
// @grant    none
// @include  https://twitter.com/*
// ==/UserScript==

const SELECTORS = `.tweet .AdaptiveMedia-photoContainer img
, .Gallery-media img
, img:not([src*="/profile_images/"])`;

function copyAltToTitle(elt) {
  if (elt.alt && !elt.title && elt.alt != 'Image') {
    console.log(`Copying alt text to title text ${elt.alt}`);
    elt.title = elt.alt;
  }
}

function copyAll() {
	document
  	.querySelectorAll(SELECTORS)
    .forEach(copyAltToTitle);  
}

copyAll();
setInterval(copyAll, 1000);