The PictureHtmlSupport enhances MediaWiki thumbnails on wiki page with <picture> elements. It does these things:

1. Wrap thumbnail <img> element with <picture> element
2. Move resposive image definition to <source> element if enabled
3. Introduce the PictureHtmlSupportBeforeProduceHtml hook for users to add additional <source> elements
4. Add a hidden <a> element next to the thumbnail HTML so web crawlers can crawl the original resolution image (https://phabricator.wikimedia.org/T54647)

1. Add the following line to LocalSettings.php:

   wfLoadExtension( 'PictureHtmlSupport' );

2. Replace includes/media/ThumbnailImage.php (make a backup!) with includes/ThumbnailImage.php from this extension.