Remember kids never get your legal advice from hn comments.
Don't become the cautionary tale kid, as crawlers like sriplaw.com will be DMCA striking your public repos eventually. =3
For example, one may use NASA public domain photos as you wish, but cannot register copyright under another license you find convenient to sue people. Also, if that public domain photo includes the Nutella trademark, it doesn't protect you from getting sued for violating Ferrero trademarks/patents/copyrights in your own use-case.
Very different than slapping a new label on something you never owned. =3