Now I posting a single photo to wall like this:
$response = $facebook->api(\"/$group_id/photos\", \"POST\", array(
\'access_token=\' => $access_to
You will need to upload each photo first with published state to false, and then use the ID's of the unpublished photos to the /me/feed endpoint to schedule the photo. The schedule needs to be within the 24 hours from the time the photos are uploaded as facebook deletes all unpublished photos in 24 hours.
Ref: https://developers.facebook.com/docs/graph-api/photo-uploads/