1. Introduction
With the development of social networks, more and more people are willing to share images to express their feelings via social network platforms such as Flickr
http://www.flickr.com
and Instagrarnhttps://www.instagram.com
. For the world's largest image-sharing website Flickr, 38% of images are explicitly annotated as positive or negative emotion tags by their publishers [1]. Understanding the emotions behind images in social networks is an important research topic with big challenges. It can also benefit many applications such as image retrieval, market advertising and personalized recommendation.