Invalid img code

In my theme I have the following code to display a simple image

<img alt="Twitter" src="<?php echo get_stylesheet_directory_uri(); ?>/images/twitter.png"></a>

But with the plugin activated the code is changed to this (which is invalid and the image is not displayed):

<img alt="Twitter" src="data:image/svg+xml,%3Csvg%20xmlns='http://www.w3.org/2000/svg'%20viewBox='0%200%201%201'%3E%3C/svg%3E" data-lazy-src="https://website.com/wp-content/themes/theme-name/images/twitter.png"><noscript><img alt="Twitter" src="https://website.com/wp-content/themes/theme-name/images/twitter.png"></noscript>

The page I need help with: [log in to see the link]