text editor alters code

Hey the_digital_manager,
Thanks for your patience, but unfortunately the text field uses the WordPress function so there is not a way to disable this.
Try adding your second image as a data-url attribute like this:

<img class="circular--square" src="https://mondzorgcentrum-winschoten.nl/wp-content/uploads/2023/01/lennie-van-vulpen-1-250x250px.jpg" data-url="https://mondzorgcentrum-winschoten.nl/wp-content/uploads/2023/01/lennie-van-vulpen-2-250x250px.jpg" />

WordPress will not remove this attribute when you try to re-edit the page as long as you edit in the text tab

Then add this code to the end of your child theme functions.php file in Appearance ▸ Editor to swap the images on hover:

function swap_image_with_data_url() { ?>
  <script>
window.addEventListener('DOMContentLoaded', function() {
  (function($){
    $('.avia-testimonial-content').hover(function(e){
        $(this).find('img').attr('data-src', $(this).find('img').attr('src'));
        $(this).find('img').attr('src', $(this).find('img').attr('data-url'));
    },
    function(e){
        $(this).find('img').attr('src', $(this).find('img').attr('data-src'));
    }); 
  })(jQuery);
});
</script>
  <?php
}
add_action('wp_footer', 'swap_image_with_data_url');

Now the images will change on hover and back on mouse-out
Please see my test page in the Private Content area.

Best regards,
Mike