Ah, nevermind. You can't use image_url, but img_url. I know it's deprecated, but it seems to work with Images from the file directory.
So instead of:
section.settings.image | image_url: width: 500
You use:
section.settings.image | img_url: 'small'
You can see the sizes here:
https://shopify.dev/docs/api/liquid/filters#img_url