Various implementations for verifying that a user device is being operated by a person are described. In some implementations, a method includes receiving a challenge request. The method selects two or more first images from a first image category and one or more second images from one or more second image categories, which are distinct from the first image category. The method sends a challenge to the user device for presentation to the person. The challenge includes a challenge parameter and data describing the two or more first images and the one or more second images. The method receives a challenge response from the user device describing a user selection of at least one image from among the two or more first images and the one or more second images and determines a result of the challenge based on the challenge response and the challenge parameter.