If you want a painting based on a photograph, it MUST be a photograph the YOU have taken. Photos taken from a professional photographer are copyrighted to the photographer.
Reference photos should be large enough and clear enough to see features and details.
Lighting: Natural light is better than the color shifts that indoor lights can add. Morning, evening or a bright overcast day can be the best lighting. Flash will cause red eye, or wash out the colors, so it is best to turn it off.
Close up images seem to give the most information to paint with.
Try to plan your background. For example, a dark haired person will be set off against a lighter background but will blend into a dark background. Be careful of shadows that fall over your subject because these may cause details of your subject to be lost.
How to “pose” your subject: Many of the portraits we love the most are those of Rembrandt. His subjects were not face-on, straight at the viewer but were often three-quarter view, (turned slightly) or in profile. Get in close so your subject fills the frame of your camera. Take photos from several different angles or different poses.
Expression and Personality: Try and shoot an image of your subject that expresses the “who” of your subject. Take lots of photos to find the hidden expression of the mouth, sparkle in their eye, tilt of the head. Often I take many many photos and have only one that looks like it will be painting.
Remember that the better the photograph, the better the painting. A good photo with the subject clearly in focus with accurate colors will make a quality painting. If your image is too small, blurry, or has a different expression than what you want, creating a painting can be difficult. If the subject is deceased, I will try to work with what you have.