flickr_bookmarklet.js

(async () => {
  let id = location.pathname.split('/')[3];
  let resp = await fetch(`https://api.flickr.com/services/rest/?method=flickr.photos.getSizes&api_key=YOUR_API_TOKEN&photo_id=${id}&format=json&nojsoncallback=1`).then(r => r.json());
  let natural = resp.sizes.size.find(s => s.label === 'Medium 640');
  let double = resp.sizes.size.find(s => (+s.width) >= 1280);
  if (natural && double) {
    return prompt('html', `<img src="${natural.source}"
  srcset="${natural.source},
  ${natural.source} 320px,
  ${double.source} 2x" />`);
  } else if (double) {
    return prompt('html', `<img src="${double.source}" />`);
  } else if (natural) {
    return prompt('html', `<img src="${natural.source}" />`);
  }
  throw new Error(`trouble! couldn't find 1x or 2x images`);
})();