Retreaving all images attached to current post

From get_media_items function and his code, you can use get_children adding mime type for images:

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

That will get you an array ($images) of images attached to $post_id. In you case, if you’re inside a loop, $post_id should be equal to $post->ID

Not exactly a WP_Query implementation as you asked for, but it’s easy.

If you still need the WP_Query way, you can use your own code i think (not tested). Inside a loop:
$attachment = new WP_Query( array(
'post_parent' => $post->ID
'post_type' => 'attachment',
'posts_per_page' => -1,
'post_mime_type' => 'image'
) );