16. In a digital medium environment for identifying an unknown person in an image, a system comprising: at least one processor; and at least one non-transitory computer-readable storage medium storing instructions thereon that, when executed by the at least one processor, cause the system to: train a cluster classifier for each image cluster of a plurality of image clusters comprising images from an image gallery based on a plurality of known person instances; determine a probability that a first unknown person instance is each known person instance in the image cluster using the cluster classifier of an image cluster that corresponds to the image based on one or more characteristics of the image cluster; determine a context weight for each combination of the first unknown person instance and each known person instance from the plurality of known person instances using a conditional random field algorithm based on a plurality of context cues between one or more of the first unknown person instance and one or more known person instances of the plurality of known person instances or between known person instances of the plurality of known person instances; calculate a contextual probability based on the determined probabilities and the determined context weights; and identify the first unknown person instance as a known person instance from the plurality of known person instances with a highest contextual probability.
18. The system as recited in claim 16 , wherein the plurality of context cues comprise: a first context cue that constrains each known person instance from the one or more known person instances to occur at most once in the image; and a second context cue that: determines that a first known person instance from the one or more known person instances and a second known person instance from the one or more known person instances appear together in the images of the image gallery; and boosts a probability that the first known person instance and the second known person instance will appear together in additional images added to the image gallery.