Bridge the gap that exists between your visitor and your service or product by introducing the real world. This is a good way to get customers or visitors to relate to you on a personal level especially if it sparks an emotional response which is something that we want to elicit.
When I begin a website I look at what the website is trying to achieve and then decide on the best way to do it. If appropriate and great imagery exists, I will consider using this. A good photo in itself can project an image of professionalism but not only that, a great photo can pull a visitor into the website and make them think about the company or experience that a service may provide.
Getting a visitor to think about the service or offering behind the website means that the website has been humanised. By showing pictures of the work environment or the people that they will work with can immediately help a visitor to relate to you on a personal level. Take it to the extreme though and it can alienate. Why do we often see a picture of a female model on the "Contact" section of a website? This annoys me greatly and usually indicates that the designer or their client has produced an unthinking cliche.
A good example of personalisation can be seen in some fitness websites for personal trainers where the picture of the actual gym and the trainer can be seen, maybe doing the things that you'd expect if you hired them. It goes without saying that the image needs to be professional and not something blurry with bad lighting.