Time-varying CAPTCHA tests are provided to distinguish a human user from a computer software agent in an online application. A test generator generates an original image and a set of (X,Y) positions. The (X,Y) positions can be a random or repeating sequence. The test generator then separates the original image to form a plurality of secondary images. The secondary images are configured to the (X,Y) positions and the original image so that when the secondary images are displaced over time responsive to the (X,Y) positions at least one of the (X,Y) position reveals the original image, or a portion thereof. A test program generator generates an executable program adapted for execution on a client device. The executable program is encoded with the (X,Y) positions and the secondary images, and configured to displace the secondary images over time responsive to the set of positions.