I’ve been using Istockphoto for about three years. Each week they post a free image for registered users. Each image is available in multiple sizes with the same basic licensing agreement that accompanies purchased images.
Below is an image from this past April. A simple design that’s just right.
Flickr has a lot of photos that are free for use under the Creative Commons license. Many only require attribution of the creator.
http://flickr.com/creativecommons/