First, have a contract with her which contains a release from her for use of her image. She may not always be your girlfriend, but assuming you're the photographer, these copyrights in these photos can last beyond your lifetime and can be bequeathed to your heirs. That document should specify who owns the photos (probably her) and who owns the copyright (probably you).
Then, you should copyright the photos. Please see the link below.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.