How to define a default post thumbnail....

default_thumbnail.php

    function default_thumbnail($html) {
        global $_wp_additional_image_sizes;
        $isize = $_wp_additional_image_sizes;
        $w = $isize['post-thumbnail']['width'];
        $h = $isize['post-thumbnail']['height'];
        if (empty($html)) {
            $html = '<figure class="default-post-thumbnail">';
            $html .= '<img src="http://placehold.it/'.$w.'x'.$h.'" alt="" />';
            $html .='</figure>';
        }
        return $html;
    }