Google the term "stock exchange" and the first response that comes up is the New York Stock Exchange. However, the second is NOT the NASDAQ, nor the Nikkei, nor even the London Stock Exchange. It's a website called stock.xchng, the largest free stock photo site on the web. Seriously.
And it really is free. All you need to do is create an account and you're in.
There are nearly 400,000 images available on the site, most of them taken by experienced Pro-Am photographers. The only thing you need to pay attention to the various rights the different artists attach to the images. Most of the time all they want is credit. Not a bad deal for access to a ton of VERY HIGH QUALITY images. Check them out.