If you’re interested in the state of the art of large domain (internet) image search, then undoubtedly Google comes up over and over again.
Google Image Search, with its simple interface and reasonable results, is the de-facto consumer-grade image search engine. Offerings from competitors are actually a little more feature rich, especially the MSN Live Image Search, but don’t resonate as loudly in academia or popular usage.
As an example, compare search results for “red corvette” from the big three: Google, MSN, Yahoo. MSN nails the exploratory task: no-refresh scrolling, quick access to filters such as “photos”, “black and white”, and image size options that feel a little more usable and natural than Googles. Yahoo! attempts some categorising and support for ontologies in their interface; while not perfect, it’s a direction highly praised in cutting-edge research.