Remember that the final likeness, facial details and colors used in the skin tones will be highly dependant on the quality of the original image provided to the artist.
When selecting a photo choose one that has good lighting.
Avoid pictures where the light comes from behind or the face is in the shade.
Consider an image taken in a spontaneous moment or one that reflects the persons personality.
When in doubt send multiple pictures and the artist will work with you on choosing the right image.
