WOW! interesting!! I have seen personas that use simply the silhouette for "male" or "female". I've also seen personas that have no image at all. Personally, I think it's important to have an image. As humans we identify with other humans more than we do with statistics or blurred figures. And an image can really enhance a persona. The image of a person can really fill in a lot of details for a design team. For example, I was doing a persona of a woman who worked as a nurse, had a family, etc, etc.... By using a photo of a smiling woman with a stethescope around her neck wearing blue scrubs, I believe the client, the developers, and the rest of team have a better sense of who they are creating their product for.
They can "relate" to her because they probably know someone like her and it makes the persona seem more "real". I think that's why finding exactly the correct image is so important, AND so difficult, it has to have right feel.