1. A computer-implemented method comprising: receiving, using one or more computing devices, a challenge request to verify a person is operating a user device; selecting, using the one or more computing devices, two or more first images from a first image category based on two or more attribute values, which correspond to the two or more first images, being substantially disparate or substantially similar relative to one another, each of the two or more attribute values quantifying a visually perceptible attribute of an object depicted by a corresponding image of the two or more first images; selecting, using the one or more computing devices, one or more second images from one or more second image categories based on one or more attribute values, which correspond to the one or more second images, being substantially disparate or substantially similar relative to the two or more attribute values corresponding to the two or more first images, the one or more second image categories being distinct from the first image category, each of the one or more attribute values quantifying a visually perceptible attribute of an object depicted by a corresponding image of the one or more second images; sending, using the one or more computing devices, a challenge to the user device for presentation to the person, the challenge including a challenge parameter and data describing the two or more first images and the one or more second images, the challenge parameter requiring the person to categorize or sort one or more images from one or more of the two or more first images and the one or more second images; receiving, using the one or more computing devices, a challenge response from the user device describing a user interaction with at least one image from among the two or more first images and the one or more second images; and determining, using the one or more computing devices, a result of the challenge to be successful based on the challenge response and the challenge parameter.