1. A system, comprising: a private data store of images previously categorized based on recognition by a human; and at least one processor and at least one computer-readable storage medium storing instructions executable by the at least one processor to implement a human interactive proof (HIP) component configured to display one or more images from the private data store as part of an HIP challenge, wherein the private data store is available to the HIP component on the basis of an arrangement to display at least one advertisement associated with the private data store, the HIP component is configured to display the at least one advertisement in conjunction with the HIP challenge, and the HIP component is configured to make a determination regarding whether a response to the HIP challenge indicates a human responder or not, based on receiving a classification of the one or more images in response to the HIP challenge.