Idomoo provides a storage solution right in the Storybuiling Suite. For most needs Manage Media or Library Assets provide what you need. However, if you want to work with a large number of files, or if you don't want to upload material to Idomoo servers, this might not be a good solution for you.
Instead, you can bring your own storage. Wherever it is. As long as you don't need credentials to access it, or that you can embed the credentials in the URL. Working with Amazon AWS S3 creates the best solution as communication with Idomoo servers is quickest. Amazon do provide a free tier, so this can cost you nothing, potentially.

Some Storybuilder features such as mapping rely on the assets being held on Idomoo's storage. You can circumvent all such limitations with the Edit Path feature. Learn more about the Edit Path feature here.