1. A method comprising: receiving a request to validate a user to determine if the user is a machine or a human; randomly selecting a set of images, wherein each image is categorized into a different category according to content of the image; determining, by a computer, the category associated with one of the set of images; generating, by the computer, a challenge instruction instructing the user to select an image from the set of images corresponding to the determined category; providing for display, via a user interface, of the challenge instruction and the set of images to the user without providing a control image for comparison; providing for receipt, via the user interface, of a user input indicating an image selected by the user among the set of images; and when the image selected by the user is associated with the category identified in the challenge instruction, generating a response to the request to validate the user, the response indicating that the user has successfully responded to the challenge instruction.