FreeDigitalPhotos: This is the first website I turn to when I need a picture for an article. They have thousands of professional photos and illustrations to choose from. Every single picture is free and available to use commercially (except at Zazzle and CafePress). You can get a small version (usually around 268 X 400) to use for free and the larger ones start at just $3. If you choose to use the free version, you'll need to attribute the photo. But they make that easy by giving you the attribution html to copy and paste.
This truly is a great site! Look at the quality of their pictures below and then go check them out at freedigitalphotos.net. You don't even need to register!