US Patent 7917508 Image repository for human interaction proofs

A method of populating an image repository is disclosed. The method includes identifying a keyword from a dictionary and querying an image source using the keyword, thereby yielding a set of images associated with the keyword. The method also includes performing a second query on the image source using a combination of the keyword and one or more additional terms, thereby yielding a plurality of images associated with the combination. Then the plurality of images associated with the combination is subtracted from the set of images associated with the keyword, thereby yielding a difference set of images which are added to the image repository if the difference set of images satisfies predefined criteria.