Search on the Web is still mostly text-based. If you want to search inside the actual content of photos, video, or audio (not just their filenames or embedded data), you're out of luck, at least on Google. New tools are emerging, though, that can scan within media files. The latest that I've heard of is Podzinger, a service that does speech-to-text conversion of podcasts and then allows you to search for spoken words and phrases. Other tools in this category: Blinkx and the soon-to-be-released Riya (rumored to be a Google acquisition target). Also, aviation buffs may have noticed that the airplane picture site Airliners.net has a "more like this" picture search.