14. The system of claim 13 , wherein the relevance score threshold is an upper relevance score threshold and the threshold level of visual similarity is an upper similarity threshold.
13. A system, comprising: a first data storage device storing relevance data for an image search result relative to a search query, the image search result referencing an image that has been identified as responsive to the search query based on a relevance score for the image search result, the relevance score being determined independent of a visual similarity of the image to other images that are referenced by other image search results that are identified as responsive to the search query; a second data storage device storing image similarity data that is indicative of a relative visual similarity between the image and the other images; and an image search subsystem comprising one or more processors coupled to each of the first data storage device and the second data storage device to generate an adjustment factor for the image search result relative to the search query based on the relevance data and image similarity data, the adjustment factor representing a quality measure for the image relative to the search query, the image search subsystem being further operable to: determine that the image search result is a co-relevant result, the co-relevant result being a responsive image search result for which: the relevance score meets a specified relevance score threshold; and the image similarity data indicates that the image referenced by the responsive image search result has a least a threshold level of visual similarity to a threshold number of the other images referenced by the image search results; and scaling the adjustment factor for the image search result by an amplification factor in response to determining that the image search result is a co-relevant result.