Hi,

Non logged in users don't have access to the Wordpress media library. 

This protection is put in place by Wordpress itself. We'll see if we can do something about this in the next version of the calendar but generally speaking it's not good idea anonymous users to upload files to your server.

I could suggest you to add this hook in your theme or child-theme functions.php file:

add_action('wp_head', function () {
    if (false === is_user_logged_in()) {
        ?>
        <style>
            .stec-builder-add-media-photo { display: none !important; }
        </style>
        <?php
    }
});

it will hide the add image button if the user is not logged-in.