1. A computer system providing a dynamic definitive image service, the system having a processor and a memory, the memory having components executing on the processor and comprising: an image request component that receives a query from a user for a definitive image that is representative of a topic of the query; an image mapping component that maps one or more terms of the query to one or more images stored in an image data store for those of the one or more images that are related to the one or more terms; an image ranking component that ranks the one or more images according to a relative likelihood of being the definitive image for the topic of the query, the image ranking component outputting a ranked definitive image; an editorial override component that allows a human editor to replace the ranked definitive image selected by the image ranking component with an editor-chosen definitive image; an image selection component providing as output the definitive image selected from the ranked definitive image and the editor-chosen definitive image; a retrieval component that retrieves an image binary associated with the definitive image selected by the image selection component; and an image response component that responds to the query with the retrieved image binary.