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Image spam is an email spam that embeds text content into graphical images to bypass traditional spam filters. The majority of previous approaches focus on filtering image spam from client side. To effectively detect the attack activities of the spammers and fast trace back the spam sources, it is also essential to employ cluster analysis to comprehensively filter the image emails on the server side. In this paper, we present a nonnegative sparsity induced similarity measure for cluster analysis of spam images. This similarity measure is based on an assumption that a spam image should be represented well by the nonnegative linear combination of a small number of spam images in the same cluster. It is due to the observation that spammers generate large number of varieties from a single image source with different image processing and manipulation techniques. Experiments on a spam image dataset collected from our department email server demonstrated the advantages of the proposed approach.