A hidden field on the new form with the album_id should work. Start by viewing the source of the form. Is the hidden field present and does it have the correct value? If that looks ok, then review your controller create action and finally your model to determine a what point the album_id is being removed.
On a side note unrelated to your problem your photo controller index action could be changed to this:
@photos = @album.photos.paginate(:all, :per_page => 9, :page => params[:page])
(Assuming album has_many photos in your model)
Aaron