How to add photos for contacts in iPhone 6s?
Unless I'm missing something from your question, all you need to do is:


Launch the Contacts app
Select a contact
Tap on Edit (at top right)
Now the circle at top left will show the add photo text
Tap on add photo
A prompt will appear with three options:


Take Photo
Choose Photo
Cancel

Select Take Photo if you want to take a photo to use, or select Choose Photo if you want to pick one from your Photos app.
The rest should be self-explanatory


If I've misunderstood your question, let me know.