Как получить путь (thumbnail)?

<?php
$upload_dir = wp_upload_dir();
var_dump( $upload_dir );

/*
    Array (
        'path' => /path/to/wordpress/wp-content/uploads/2018/12
        'url' => https://example.com/wp-content/uploads/2018/12
        'subdir' => /2018/12
        'basedir' => /path/to/wordpress/wp-content/uploads
        'baseurl' => https://example.com/wp-content/uploads
        'error' =>
    )
*/

echo $upload_dir['url']; // Вот это наверное вам надо