1. A method comprising: receiving an example image for use in querying a collection of digital images; using, by a processing system, a local feature descriptor representing a portion of the contents of a digital image and a global feature descriptor representing substantially all of the contents of the digital image to perform a content-based image comparison of the collection of digital images with the example image, to automatically rank the collection of digital images with respect to similarity to the example image, wherein the global feature descriptor is content based and not keyword based; and using a final classifier and multiple different intermediate classifiers to perform the automatic ranking, comprising: generating, by the different intermediate classifiers, intermediate relevance metrics with respect to different modalities; and generating, by the final classifier, results from the intermediate classifiers into a final relevance metric to display images in ranked order.