October CMS - extend page to have an associated image

What are the best and clean possible ways to extend functionality for including an associated image to each page object (CMS page or StaticPage), adding a corresponding upload widget to the page's configuration in the OctoberCMS backend?

(Additionally, and just as an aside: In what way can one add an associated image to a site's theme?)

For CMS pages, add a register() method to your Plugin.php file:

public function register()
{
    Event::listen('backend.form.extendFields', function($widget) {
        if (!$widget->model instanceof \Cms\Classes\Page) return;

        $widget->addTabFields([
            'settings[image_filet]' => [
                'label' => 'Page image',
                'tab' => 'Image',
                'field' => 'settings[image_file]',
                'type' => 'mediafinder',
                'comment' => 'Image url is available via the page settings under the name of image_file'
            ],
        ], 'primary');
    });
}

And the value is available via this.page.image_file Twig value