Photos
There are dozens of companies that provide images either free or on a royalty basis. A Google search on terms like "stock photo" should get you plenty of links to follow up. Here are a few to start with:
- Fotosearch Stock Photography Search
- Awesome graphics for PowerPoint
- Can Stock Photo
- Creative Commons is a source for royalty-free images
There are also images, clip-art and media available under the Creative Commons license.
See PowerPoint MVP David Marcovitz' site for more information about Creative Commons licensing and for a list of resources.