Get an array containing ONLY post IDs of attached images

The following code should work for you,

$images = get_children( array('post_parent' => $post_id, 'post_type' => 'attachment', 'post_mime_type' => 'image') );
$image_ids = array();
if($images){
   foreach( $images as $imageID => $imagePost ){
      array_push($image_ids, $imageID);
   }
}

The array $image_ids would get you the image IDs.