How can I use NID Token with the Insert Module?
You want to use the filefields_path module to add node tokens to your imagefield (or any other file fields): 


  The File (Field) Paths module extends the default functionality of
  Drupal's core Upload module, the FileField module and many other File
  Upload modules by adding the ability to use node tokens in destination
  paths and filenames.


In my test environment with the insert module and this module enabled, I now have the capacity to use node:nid in the filefield path as well as a number of other useful tokens: