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.