In today's age of digital multimedia deluge, a clear understanding of the dynamics of online communities is capital. Users have abandoned their role of passive consumers and are now the driving force behind large-scale media repositories, whose dynamics and shaping factors are not yet fully understood. In this paper we present a novel human-centered analysis of two major photo sharing websites, Flickr and Kodak Gallery. On a combined dataset of over 5 million tagged photos, we investigate fundamental differences and similarities at the level of tag usage and propose a joint probabilistic topic model to provide further insight into semantic differences between the two communities. Our results show that the effects of the users' motivations and needs can be strongly observed in this large-scale data, in the form of what we call Kodak Moments and Flickr Diamonds. They are an indication that system designers should carefully take into account the target audience and its needs.