Hi,

Yes, via filter.

Open your theme or child-theme functions.php file and place following hook:

add_action('wp_footer', function() {
    if ( !wp_script_is('stec-js') ) {
        return;
    }
    ?>
    <script type="text/javascript">
        (function ($) {
            /* CONFIG */
            const IMAGE_SRC = 'https://stachethemes.com/wp-demos/the-calendar/wp-content/uploads/sites/13/2018/09/sas.jpg';
            const THUMB_SRC = 'https://stachethemes.com/wp-demos/the-calendar/wp-content/uploads/sites/13/2018/09/sas-300x137.jpg';
            /* END OF CONFIG */
            $(function () {
                $.stecExtend(function (m) {
                    if (m.glob.options.view === 'boxgrid') {
                        const events = [];
                        $.each(m.calData.filterGetEvents, function () {
                            const event = {...this};
                            if (event.images.length <= 0) {
                                event.images.push(0);
                                event.images_meta.push({
                                    alt: "",
                                    caption: "",
                                    description: "",
                                    id: 0,
                                    src: IMAGE_SRC,
                                    thumb: THUMB_SRC,
                                    title: ""
                                });
                            }
                            events.push(this);
                        });
                        m.calData.filterGetEvents = events;
                    }
                }, 'beforeReturnGetEvents');
            });
        })(window.jQuery);
    </script>
    <?php
});

Just change the images urls to your desired image where it says "CONFIG..."