I thought I'd give a free plug to Getty Images, the world's leading provider of imagery, film and digital services (to give you some idea of their size: they offer over 70 million images and 30,000 hours of film). The company has recently launched a new (free) service that you may find useful. If you want to locate a particular image but can't quite see yourself wading through a million (plus) images to uncover it - you can ask a Getty researcher to do it for you. Let's face it - they know what they have on file better than anyone else!