Useful photos for portraits usually have simple lighting and have small smiles rather than big smiles.

A great photo is not always good for a drawing or painting.

No huge grin

A big grin is seldom good for a painting. Sometimes visible teeth are ok. Subtle smiles are best. Look at some great portraits from museums: You will seldom see a big smile in a painting in a museum. Frans Hals is a wonderful exception to this rule.

Simple lighting

Look at the portrait drawings page

Lighting should be strong and directional.

Sometimes natural light is good coming from a single window.

Sometimes sitting people near a table lamp works (with or without the shade). 

People sometimes look better from the side.  Sometimes a turn in the neck is good.  Sometimes eyes to the camera is better, and sometimes eyes to the side looks better.

Take lots and lots!

I take hundreds of pictures when I am taking photos for a portrait.