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.
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.