image provider: base id is not updated when replacing thumbnail image on creation

Normally when the thumbnail field is updated the provider makes sure that the id is synced back to the base_id in the atom. This is done by implementing the update_atom hook. However on creation the system only calls the register_atom hook. So the image provider should implement both hooks and not just only the update, otherwise it is possible to break the atom at creation if the thumbnail image is replaced on the options form.