How can I add a CSS class to an <img>
tag by using a template file if output looks like this:
<!-- BEGIN OUTPUT from 'themes/contrib/bootstrap/templates/system/image.html.twig' -->
<img class="media__image media__element b-lazy b-responsive img-responsive b-loaded" src="/sites/default/files/styles/media_image/public/2018-03/Trainer-Norbert-Sauer-auf-der-Rennbahn-Wambel-Am-Donnerstag-249903.jpg?h=c913b3c6&itok=sENPOWtc" alt="Trainer Norbert Sauer auf der Rennbahn Wambel">
<!-- END OUTPUT from 'themes/contrib/bootstrap/templates/system/image.html.twig' -->