1. A method of selecting test stimuli for use in evaluating performance of a watermarking algorithm, the method comprising the steps of: calculating an image complexity rate (ICR) for each of a plurality of test stimulus candidates; classifying said plurality of test stimulus candidates into L (where L is an integer more than or equal to 2) levels of the test stimuli, based on the calculated ICR values; and selecting at least one test stimulus from each of the L levels of the test stimuli.