1. A computer system for providing a dynamic definitive image service, the system comprising: a processor and memory configured to execute software instructions embodied within the following components; an image data store that stores one or more images or references to images stored in other data stores; an image request component that receives a request during a query for a definitive image that is representative of a topic of the query; an inverted index component that maintains an inverted index that maps one or more query terms to one or more images that are related to those terms; an image ranking component that ranks one or more images identified in the index that potentially match the query terms to sort identified images by a relative likelihood of being a definitive image for the topic of the query; an editorial override component that provides an interface through which a human editor can manually override the definitive image selected by other components of the system to identify an editor-chosen definitive image; an image selection component that selects a definitive image based on images returned from the index, any overridden editor-chosen definitive image, and ranking input from the image ranking component; a thumbnail retrieval component that retrieves an image binary associated with the selected definitive image based on a definitive image identifier provided by the image selection component; and an image response component that responds to the received image request with the retrieved image binary.
3. The system of claim 1 wherein the image request component operates on top of an existing search engine to service requests for definitive images in addition to requests for image or other types of searches already handled by the search engine.