1. A computer-implemented method of determining image characteristics, the method comprising: obtaining, by the one or more computing devices, one or more first images having an unknown characteristic and one or more second images having a known characteristic; providing, by the one or more computing devices, the one or more first images and the one or more second images to a plurality of user devices in a verification challenge, the verification challenge comprising one or more instructions to be presented to a user of each user device, the instructions being determined based at least in part on the unknown characteristic of the one or more first images and the known characteristic of the one or more second images; receiving, by the one or more computing devices, a response to the verification challenge from at least a subset of the plurality of user devices; determining, by the one or more computing devices, the unknown characteristic of the one or more first images based at least in part on a plurality of the received responses; providing, by the one or more computing devices, data indicative of the determined characteristic to one or more machine learning models; and training, by the one or more computing devices, one or more machine learning models based at least in part on the data indicative of the determined characteristic.