Public domain (free) pictures for your website - MorgueFile doesn't have a huge selection of professional quality pictures, but the pictures it does have are free for use for any purpose. The site isn't trying to sell anything, so there are no annoying "download hi-res yada yada" by each pic.
For a list of free photo sites, you can check out the wikipedia page on it:
http://en.wikipedia.org/wiki/Wikipedia:Public_domain_image_resources