In this article, we propose a spam image clustering approach that uses data mining techniques to study the image attachments of spam emails with the goal to help the investigation of spam clusters or phishing groups. Spam images are first modeled based on their visual features. In particular, the foreground text layout, foreground picture illustrations and background textures are analyzed. After the visual features are extracted from spam images, we use an unsupervised clustering algorithm to group visually similar spam images into clusters. The clustering results are evaluated by visual validation since there is no prior knowledge as to the actual sources of spam images. Our initial results show that the proposed approach is effective in identifying the visual similarity between spam images and thus can provide important indications of the common source of spam images. Copyright © 2009, IGI Global.