1. A method for searching a repository of documents using an image as a search query, the method comprising: receiving a query image I* to be used to search a repository of documents; using a hash function to hash said query image to obtain a hash value h(I*); setting, by a user, an initial threshold level τ 0 for said search, said initial threshold being based upon a visual examination of said query image; determining a tolerance level τ for said search, where τ=τ 0 +ƒ(D) and ƒ(D) is a non-negative and monotonically increasing function of a measure of distortion D where 0≦D≦D max , said tolerance level defining a possible match for said search; for each document in said repository: for each image in said document: extracting said image I k from said document; hashing said extracted image to obtain a hash value h(I k ) for said extracted image; computing a distance d between said hash value h(I*) and said hash value h(I k ) where d=∥h(I*)−h(I k )∥; comparing distance d against said tolerance level τ; and flagging said document as a possible match in response to said distance being less than said tolerance level; and providing any of said flagged documents to a user in response to said search.