In my opinion, the less stock photos you use, the better. Even with the options available from large stock libraries like istockphoto.com, the web seems to be full of stock-junk. It reminds me of the days when every second website you visited was a FrontPage template.
Visitors to your website can smell stock photos from a mile away and will sense the negative connotation of a shortcut and/or misrepresentation. Professional photos are original, authentic and a true representation of your company, people, products and services. They do not misrepresent.
The Red Piano on Bourbon Street, West Edmonton Mall, had the option of using stock photos on their new website. Although more expensive, they decided to go with professional photography. I think it was a good decision, see for yourself: