Thumbnails in widget (WP Favorite Posts)

Hello!
Help me please.
I use the plugin WP Favorite Posts.
I would like to add a thumbnail to each Favorited item in the widget, I’ve tried the below code:
echo get_the_post_thumbnail( $o->post_id, thumbnail);
In line 210 in the wp-favorite-posts.php

function wpfp_list_most_favorited($limit=5) {
    global $wpdb;
    $query = "SELECT post_id, meta_value, post_status FROM $wpdb->postmeta";
    $query .= " LEFT JOIN $wpdb->posts ON post_id=$wpdb->posts.ID";
    $query .= " WHERE post_status='publish' AND meta_key='".WPFP_META_KEY."' AND meta_value > 0 ORDER BY ROUND(meta_value) DESC LIMIT 0, $limit";
    $results = $wpdb->get_results($query);
    if ($results) {

        echo "<ul>";
        foreach ($results as $o):
            $p = get_post($o->post_id);

            echo "<li>";
            echo "<a href='".get_permalink($o->post_id)."' title='". $p->post_title ."'>" . $p->post_title . "</a> ($o->meta_value)";
            echo "</li>";
        endforeach;
        echo "</ul>";
    }
}

But for some reason it doesn’t work.
Help me please