I can think of two approach (if you don't care about any special handling of image)
1) use WebDAV and put all asset in one instance
2) use multiple CQ instance to upload asset using webdav and then using package manager or replication transfer them across. In this case disable DAM workflow while transfer and use merge mode for packages.
CQ5.5 also has feature of delegation but unfortunately never used it.
Yogesh
