How to get only current images of a post

Hmm… Try altering your query a little to:

$args = array(
    'post_parent' => $post_id, 
    'post_status' => 'inherit', 
    'post_type' => 'attachment', 
    'post_mime_type' => 'image',
    'order' => 'ASC', 
    'orderby' => 'menu_order ID',
);
$attachment = new WP_Query( $args );
$attachment = $attachment->posts;

Alternatively, you could use the following:

$attachments = get_children( 
    array( 
        'post_parent' => $post_id, 
        'post_status' => 'inherit', 
        'post_type' => 'attachment', 
        'post_mime_type' => 'image',
        'order' => 'ASC',
        'orderby' => 'menu_order ID',
    )
);