Any fair to good quality picture will do. Pictures tend to become smaller in size due to the process so bigger the format, better the quality of finished pic. For the best most realistic results have a quarter pic(picture taken of side and front of face, slight angle) it shows the face view everyone will see instead of just front or just profile view, these are necessary when used clinically for actual FFS. Have at it, you might like the result, make sure background of the pic is neutral or of solid color. If not it just makes it little harder to do however it is still doable.
Love Alexia.