What is a “primary photo”? It’s your photo standing out of the rest, and it’s the primary reference of you for the artist to make your gorgeous portrait. It’s the photo in which you like yourself and your look (your hairstyle, foreshortening, etc.) the most. The face should be the main focus of this photo and should be seen clearly.
A slight hint: your image will vary depending on how close you position the camera. Keep this in mind when choosing your photo.
If you take a selfie, check if the photo is mirrored. It’s not always possible to understand it if there are no inscriptions on the picture. Otherwise, the artist may depict the features of your appearance (hair parting, bang position, piercing, birthmarks, etc.) from the wrong side.