Why is the featured image of my post included in the photo gallery?

@dandelion222 this is by default in Enfold because the lightbox function is missing the .gallery class in the groups property. The script tries to catch the closest group (css class) wrapped around the image that has been clicked on. Unfortunately without .gallery this is .post-entry. The jQuery script will than pick up all images within the div with class .post-entry. You can verify this by adding some more single images to the post content. They all will appear in the gallery popup.

If you have a child theme you could use a monkey patch in a custom jquery script:

(function($){

    var _old_avia_lightbox = $.fn.avia_activate_lightbox;
    $.fn.avia_activate_lightbox = function( variables ){
    if ( ! variables ) {
        var variables = {};
    }

    variables.groups = ['.gallery', '.avia-slideshow', '.avia-gallery', '.av-instagram-pics', '.portfolio-preview-image', '.portfolio-preview-content', '.isotope', '.post-entry', '.sidebar', '#main', '.main_menu'];

    return _old_avia_lightbox.call( this, variables );

    };

})(jQuery);

Put the code above in a file called mmx.js and put the file in the child theme directory. In the functions.php file of your child you would than add the following lines:

add_action( 'wp_enqueue_scripts', 'mmx_load_scripts', '99' );
function mmx_load_scripts() {
    wp_enqueue_script( 'mmx', get_stylesheet_directory_uri() . '/mmx.js', array( 'jquery', 'avia-default' ) );
}

@dandelion222 it is always difficult to say whether an other gallery plugin will do the job. It may even conflict with enfolds script. My advice: if there is a way to get it done in enfold (like my monkey patch) do it. Each additional plugin is a new potential source of trouble ;-)