how to delete pictures after you have synced

Only photos taken with the phone or saved from emails/websites can be deleted directly on the phone - photos that were synced from your computer are deleted by moving/removing/de-selecting them from where they were synced from on your computer and then re-syncing.

The newest version of iTunes does not have this option, how do I delete synced photos from my iPhone 6s?

If you synced them from a computer then they are still removed in the same, by changing your sync selection on the Photos tab when connected to your computer's iTunes