In a broad sense, public domain images are images that the general public are permitted to use without restriction because no exclusive copyright restricts their usage (see also: Using Copyrighted Images). These images are usually quite old (expired copyright), created by the government, or released into the public domain by their creator. For a more comprehensive explanation, see Wikipedia's Public Domain article.
Star Army allows for the use of public domain images. Any unmodified Public Domain image must be placed in the public_domain: media namespace.
For example, this image is by NASA and is public domain:
Star Army is not a repository for images; public domain images should only be on Star Army if they're actually being used in articles.