Social events are very common activities, where people can interact with each other. During an event, the organizer often hires photographers to take images, which provide rich information about the participants' behaviour. In this work, we propose a method to discover the social graphs among event participants from the event images for social network analytics. By studying over 94 events with 32,330 event images, it is proven that the social graphs can be effectively extracted solely from event images. It is found that the discovered social graphs follow similar properties of online social graphs; for instance, the degree distribution obeys power law distribution. The usefulness of the proposed method for social graph discovery from event images is demonstrated through two applications: important participants detection and community detection. To the best of our knowledge, it is the first work to show the feasibility of discovering social graphs by utilizing event images only. As a result, social network analytics such as recommendations become possible, even without access to the online social graph.