1. An image similarity scoring method comprising: counting a number of matching visual words between a query image and a candidate image, wherein each matching visual word is found in both the query image and the candidate image; generating an image similarity score for the query image and the candidate image based at least in part on the counted number of matching visual words; and generating a normalized image similarity score based at least in part on the image similarity score and one or more of a complexity of the query image and a complexity of the candidate image, wherein the normalized image similarity score is generated according to a weighting function that is described by a finite unimodal function.