你可能想看看这个插件Regenerate Thumbnails by Viper007Bond.

基本上，这就是如何做到这一点：

function regenerateThumbnails() {

    $images = $wpdb->get_results( "SELECT ID FROM $wpdb->posts WHERE post_type = \'attachment\' AND post_mime_type LIKE \'image/%\'" );

    foreach ( $images as $image ) {
        $id = $image->ID;
        $fullsizepath = get_attached_file( $id );

        if ( false === $fullsizepath || !file_exists($fullsizepath) )
            return;

        if ( wp_update_attachment_metadata( $id, wp_generate_attachment_metadata( $id, $fullsizepath ) ) )
            return true;
        else
            return false;
    }
}