Go to your theme functions.php file and add following filter:

add_filter('stec_builder_front_elements', function($elements) {
    ob_start();
    ?>
    <button type='button' class='stec-builder-element-content-button-style stec-builder-submission-add-image'>
        <i class="fas fa-plus"></i>
        <span><?php esc_html_e('Image', 'stec'); ?></span>
    </button>
    <span class="stec-builder-submission-add-image-src stec-builder-element-content-p-style"></span>
    <?php
    $elements['image'] = array(
        '<div {{params}}><input class="stec-builder-hide-upload" name="stec_public_file_image" type="file" accept="image/png, image/jpeg"/>' . ob_get_clean() . '</div>',
    );
    return $elements;
});

This will show the upload image button at all times.

However, you will have to modify the core code because  there is check in place if the user is logged-in it will only look for media images and will not check for upload files.

Open file:

stachethemes_event_calendar\assets\php\class.submit-event.php

on line 154 starts this code:

        if (is_user_logged_in()) {
            if (Admin_Helper::isset_post('images')) {
                $event->set_images(Admin_Helper::post('images', array(), FILTER_VALIDATE_INT, FILTER_REQUIRE_ARRAY));
            }
        } else {
            if (isset($_FILES['stec_public_file_image'])) {
                $image_id = self::image_file_upload($_FILES['stec_public_file_image']);
                if (filter_var($image_id, FILTER_VALIDATE_INT)) {
                    $event->set_images(array($image_id));
                }
            }
        }

Replace it with:

if (isset($_FILES['stec_public_file_image'])) {
    $image_id = self::image_file_upload($_FILES['stec_public_file_image']);
    if (filter_var($image_id, FILTER_VALIDATE_INT)) {
        $event->set_images(array($image_id));
    }
} elseif (is_user_logged_in() && Admin_Helper::isset_post('images')) {
    $event->set_images(Admin_Helper::post('images', array(), FILTER_VALIDATE_INT, FILTER_REQUIRE_ARRAY));
}

I'll make this change in the next version as well so you won't have to do it again.