twitter-img-alt.js

// Bookmarklet: display the alt text of images on Twitter
// Instructions: copy the whole text block, starting at `javascript:` and make it the address of a bookmark
// You can test it here: https://twitter.com/rauschma/media

javascript:{
  let img;
  
  const match = /\/photo\/([0-9]+)$/.exec(String(location));
  if (match) {
    const photoNumber = match[1];
    img = document.querySelector(`li[role=listitem]:nth-child(${photoNumber}) img[alt]`);
  }
  
  if (!img) {
    img = document.querySelector('a[href *= "/photo/"] img[alt]');
  }

  const alt = img ? img.alt : '(No <img alt> found)';
  alert(alt);
}