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 a plurality of L levels of the test stimuli, based on the calculated ICR values, where L is an integer greater than 2, and wherein each of the plurality of L levels has a predetermined ICR range, and wherein the ICR is calculated using the following Equation: U = - 1 * - 1 w × h ⁢ ∑ x = 1 w ⁢ ∑ y = 1 h ⁢ [ f ⁡ ( x , y ) - f _ ⁡ ( x , y ) ] 2 where f(x, y) is the intensity-level at pixel (x, y) and f (x,y) is the average gray-level in a 3×3 window centered at (x, y) and w and h are the width and height of the image; and selecting at least one test stimulus from each of the L levels of the test stimuli.