You simply upload all assets (images, CSS, JS or any other file related to the theme of your site) to the resource folder. Then you just reference it by file name.
Subfolders are currently not possible, but I think many Shopify buyers will appreciate this. I personally like the clean folder structure.
CSS
{{ 'styles.css' | asset_url | stylesheet_tag }}
JS:
{{ 'modernizr.min.js' | asset_url | script_tag }}
Link to images or other assets in CSS:
{{ 'background.jpg' | asset_url }} div
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