The best approach to answering your question focuses on whether the other person is "using" the photographs unlawfully -- not whether the other person violated some law or terms of service to acquire the photographs.
You don't explain how the photographs are being used. That conversation needs to occur with your own intellectual property attorney in private.
Let's assume, however, that YOU took the photographs and, therefore, own the copright in the photographs. If so, and if the photographs are being published on some other website, then you may send a notice to the company that hosts that website and demand they be taken down.
What you can, and should, do depends on what the other person is doing with the photographs. Good luck.
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.
You need to consult an IP attorney and analyze all relevant facts.
If these are photos you took yourself, then you own the copyright to them, and you if you didn't give them permission to copy them, display them, etc., then they're committing copyright infringement, and you can use the site's DMCA policy to report their misuse. Read the policy carefully and follow it exactly, or the site will not remove the infringing photos from the infringer's pages.
If you APPEAR in the photos, but didn't take them yourself, then you can't use the DMCA, which is only for copyright owners. In that case, you may have a privacy claim, if these photos are only supposed to be available to your authorized site friends. See an IP lawyer for help.
Avvo doesn't pay us for these responses, and I'm not your lawyer just because I answer this question or respond to any follow-up comments. If you want to hire me, please contact me. Otherwise, please don't expect a further response. We need an actual written agreement to form an attorney-client relationship. I'm only licensed in CA and you shouldn't rely on this answer, since each state has different laws, each situation is fact specific, and it's impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.