1. A method comprising: in response to receiving a request to validate a user to determine if the user is a computer or a human, performing the following steps: selecting a set of images from an image data store, wherein each image selected is different from the other selected images; generating a challenge instruction, the challenge instruction instructing the user to select from the set of images an image identified in the challenge instruction; generating a user interface for displaying the challenge instruction and for displaying each image from the set of images to the user one at a time in a sequential fashion, each image being displayed for an interval of time; and receiving a user input indicating an image selected by the user from the set of images; wherein if the user selects the image identified in the challenge instruction while the image is being displayed, generating a response to the request to validate the user, indicating that the user has successfully responded to the challenge instruction.