Running firn new scaffolds a firn site, including the creation of a folder named static:

static
├── css
│   ├── firn_base.css # firn_base is included when a firn site is created
├── img
│   └── favicon.png
└── js
    └── main.js

These files will be transferred over to your built site when you run firn build.

The static directory is where you want to put all your CSS, JavaScript, and site-wide images (such as a favicon).

Firn does not support org-attach, but does support including file link attachments. If you want images in your org-files to appear as in your built side, you need to centralize your images in a single folder. By default, a new Firn site looks for aa directory called data, and then assumes that you are linking images into your documents from any subsequent folder under data.