While I am looking the doc. I saw the following document structure,
.
├── _config.yml
├── _drafts
| ├── begin-with-the-crazy-ideas.textile
| └── on-simplicit
A static file can be placed anywhere within the site directory that is not a collection of some sort. So not within a directory that begins with "_". What makes it static is the fact that it does not have YAML frontmatter.
Id recommend making a directory named "assets" or "images" and have them in there.
edit:
what are you trying to use the feature for?