David, I don't believe there is any "official" guidance. Let's use this discussion topic to come up with some suggested best practices which we can then place on the "description" of this project.
Here is an initial draft...
Best Practices for Image Use:
The following are guidelines to help determine best practices for profile pictures.
1) If possible include a photograph of the person.
You should upload pictures that you have taken yourself, or where you have permission from the photographer who took the picture. Or you should be reasonably sure that the photograph is in the public domain.
2) Because the main profile picture is also used in the tree view it is suggested that the picture be a "head and shoulder" shot and not the full "foot to shoulder" framed shot of the person.
3) For the main profile picture individual photos are preferred to group photos.
4) For profiles that pre-date photography a painted portrait of the person if one exists works well. Make sure the painting was done contemporaneously with the person depicted and not merely an "artistic" interpretation of a random person from that historic time period. A photograph of a sculpture of the person can also be used if one exists.
5) In all cases please include an attribution giving credit to the photographer or artist that generated the original image used for the profile picture.
6) In many cases a photograph, painting or sculpture do not exist. In those cases there are many other suitable alternatives including:
Cemetery Gravestone
Picture of the person's residence.
Picture of the person's signature.
Picture of an actual personal belonging that the person used.
Picture of a primary source document that includes vital information about the person like a birth or death certificate.
Picture of an "historical marker" relating to the profile.
Picture of the person on coin, currency or a stamp.
Picture of the Cemetery (only used if the person doesn't have a headstone).
Picture of the ship or sailing vessel used by an immigrant ancestor.
Picture of a geni project image. Projects categorize individuals based on a research criteria. Some projects will use a custom project "Icon" or "image" for all profiles that don't yet have a better image available for them.