This guide on the Mindjet web site covers how to transfer items like map parts, images and icons which you have added to the sets in the Library of previous versions of MindManager to the current version.
Download MindManager Migration Guide PDF
However I recommend placing your customised objects in a folder of your user folders and linking this as a Package Folder to MindManager in File > Options > Package Folders. I think this was automatically migrated to MindManager 2016.
This particularly important when sharing customised objects with other users but the contents of this blog also explain the mechanics of the Package Folder to any user.