1. A method comprising, by one or more computing systems: accessing an image, the image being posted on a social-networking system, wherein the image is accessed in response to an action being requested on the image on the social-networking system; extracting a predetermined number of features from the image; processing the extracted features using a machine-learning model to determine a probability that the image is an illegitimate image; extracting metadata from the image; comparing, using the machine-learning model, the extracted metadata to one or more of the extracted features; adjusting, using the machine-learning model, the determined probability that the image is an illegitimate image based on the comparison; categorizing, based on the adjusted probability, the image as a natively-captured image or as an illegitimate image; and applying one or more distribution policies to the image based on the categorization, wherein the distribution policies limit distribution of the image on the social-networking system, and wherein at least one of the distribution policies comprises prohibiting actions on the image by users of the social-networking system.