First add a class to the image tag like this (e.g. Bootstrap)
$imageTag = "$clickTag<img src='" . htmlspecialchars($imageUrl, ENT_QUOTES) . "' width='$width' height='$height' alt='$alt' title='$alt' class='img-fluid'$imgStatus />$clickTagEnd";
I changed this in the file asyncspc.php at line 3462 (folder 'delivery') but this depend on the method you are using.
Then copy this file https://raw.githubusercontent.com/revive-adserver/revive-adserver/35aa0c7c0507be3f4a94b7a7501f6100a9884b85/www/delivery_dev/async.js
and replace the file in the folder 'delivery'.
You could minify it if you like.