Wikipedia is extremely good on this topic (Imaginary: Sociology) and says pretty much what I would have said and more, identifying key sources (Castoriadis, Lacan and Taylor) that clearly show the collective and systemic nature of the imaginary and its connection to phantasy. Philosophically it is closer to post-structuralism than hermeneutics, although Habermas enters the discussion. Well worth a read, and following up the references. Social representation is much narrower and more specific - although grounded in psychoanalysis (Muscovici), it is more Durkheimian and functionalist. Social representation theory is a subdomain of social psychology with its own concepts, although linking to social constructionism, and lacks the emphasis on imagination. The divergences are often subtle though, from ontology to approaches to behaviour.

The discussion of these concepts is extremely broad and started from the foundation of the social sciences. As mentioned earlier, you can find the term social representation in the old school of functionalism, then in the psychoanalytic school and later in Cornelius Castoriadis. More recently, post-structuralism and post-Marxism have also used this term. Since the discourse analysis, it has also been used by Teun van Dijk. In my opinion, human beings share a common framework nourished by society, and that is our social imaginary. The way in which we express this imaginary produces social representations, which are different from each other because social actors tend to interpret their society according to their particular conditions and forms of socialization. In other words, as Pierre Bourdieu said, their habitus.

My friend, is a big discussion through the sociological studies. To understand better this discussion I suggest you to read the work of Manuel Baeza from the Universidad de Concepción, Chile.

Thanks Stephen; Alonso and Jesús. Your comments are very helpful.