There are two places where you should keep images within SilverStripe:
tutorial/images/ – Only designer can access and manage images.
assets/ – Content author can upload and manage using SilverStripe.
Each of these folders can naturally have subfolders to better organise the items.
This practice also applies to other assets such as multimedia files and documents.
In the case of this tutorial, our images are a core part of the layout so we put them in the tutorial/images folder, away from the control of the CMS user.