Photo websites
Flickr
Flickr is Yahoo's online photo storage site. Photos are free, but you can only use images which have the appropriate creative commons licence (see below for more detail).
Sport England photo library
This is a collection of over 600 high-quality free photos. I developed this for Sport England to help people working in sport to improve their marketing and promotion. Many of the photos are available in very high resolution.
Istockphoto
A fantastic low-cost library of royalty free images that cost from £1-£12 depending upon the size and resolution needed. You can search by keyword, subject, color, and white space.
Stock Exchange
More than 350.000 photos available. Free for use under certain conditions.
Getty images
This is where the pros go. The photos are great, but the prices are higher than on the other sites. Expect to pay between £75 and £200 per image.