You have copyright in your photograph, but, as odd as it sounds, you do not have the legal right to sue someone for infringement of your photograph unless and until it is registered. You can find a lot of basic information at the link I have supplied.
You first need to answer the question of why their use of your photograph causes you so much distress.
Is it a photograph that you have sold or licensed, or intend to sell or license, to others? Does the photograph have some particular importance to you personally?
If no to either, and if you are seeking a legal remedy solely out of a sense of righteous indignation, then not only should you reconsider the value and wisdom of that motivation you also need be aware that even if the publication of your photograph is an infringement of its copyright you have suffered essentially NO damage.
As noted previously, registration of the copyright in your photograph is a prerequisite to filing a lawsuit against the infringers (your only real leverage in this situation). Visit copyright.gov to learn how to register the copyright in photographs. Just as importantly, however, because you did not register the copyright in your photograph before the infringement occurred you are not entitled to recover the statutory damages that are provided by law -- in short, at trial you would be obligated to prove the "actual damages" that you suffered by reason of the photograph's publication. So your answer to the question of why are you so distressed is legally important.
Copying someone else's photograph and using it without permission is, most of the time, simply wrong. But there are many wrongs in life that cannot be righted or are just not worth the time and money to make right. This may be one of them. Please note that if you contact the person who published your photograph and accuse him or her of infringement then that person can file suit against you to seek a declaration as to who is the rightful owner of the photograph and/or its copyright. Before you contact that person (if that's what you decide to do) then you first need to discuss that correspondence with an attorney.
You should discuss the specific details of your circumstances with an Intellectual Property Lawyer. This area of law is very tricky and requires a through analysis of your unique circumstances. You may also be able to explore other legal rights outside "copyright" depending on the factual circumstances.
Internet privacy rights are in a state of flux. Federal rulings and state rulings by the various state court systems provide some guidance, but each situation is very fact specific. Without a great deal more information about the particulars in your situation, a meaning answer is not possible. I agree with Mr. Dadmehr that this area of the law is tricky and is very, very fact specific.
A good number of commentators suggest that the internet is a public forum with little privacy options available. Inroads to adopt and enforce privacy rights are being made, however.
A public picture of you taken in a public place and posted on the internet that causes no harm to you is quite different than an intimate picture of you taken in a private place that is posted publicly on the internet and causes you harm.
Contact a local lawyer that will provide a consultation to you so you might explain your situation in more detail.
Good luck to you.
NOTE: This answer is made available by the lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed professional attorney in your state. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question.