Add class to featured image?

I am trying to use Cloudinary to make my page responsive and create breaking points for images on mobile. To do this I need to set the img tag parameters.

<img
data-src=”https://res.cloudinary.com/demo/image/upload/w_auto,c_scale/smiling_man.jpg&#8221;
class=”cld-responsive”>

I tried adding cld-responsive to CSS Style section, and possibly if this is valid method my JS on the page may have issues.

I love your plugin so-far, but I need to figure this out to not serve huge featured images to mobile.

BTW, Cloudinary cites using a plugin like yours to use their images as WP featured images.