The Cloud computing emerges as a new computing paradigm, which provides a reliable, flexible, QoS guaranteed IT infrastructure and services. In this context, users upload Virtual Machines (VMs) into a Cloud storage service, from which they are propagated on demand to the physical nodes on which they are supposed to run. It is therefore important for the Cloud storage service to provide efficient support for VM storage in a context where a large number of clients may concurrently upload a large number of VMs, each of which may subsequently be needed by a large number of computing nodes. This paper addresses the problem of building such an efficient distributed repository for Cloud Virtual Machines . To meet this goal, our approach leverages BlobSeer, a system for efficient management of massive data concurrently accessed at a large-scale as a storage back-end for the Cloud VM repository. As a case study, we consider the Nimbus Cloud environment, whose repository currently relies on the GridFTP highperformance file transfer protocol. The research conducted so far, and a prototype has been experimented on the Grid'5000 testbed.