A banner image should be something that speaks to your clientele. If your clients are generally in a certain age group, using images of people in the same age group would be best. If your practice is focused on clients from a specific city or location, using images in and around that location will show clients and prospects that you share this communicate you and face the same issues that they face. If chosen well, the banner image should go a long way in making a good first impression and building rapport.