You gave up the picture willingly, although not for this purpose. You do not say what sort of picture you sent. You can hire an attorney to write a letter about taking it down (cease and desist), and that might work. What do you mean as to harassment? Can you just solve that by having no further contact? You may want to detail the facts with an attorney.
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There is nothing you can do... you lost control of the photo once you sent it to the first person.
I am a former federal and State prosecutor and now handle criminal defense and personal injury/civil rights cases. Feel free to check out my web site and contact me at (212) 385-8015 or via email at Eric@RothsteinLawNY.com. The above answer is for informational purposes only and not meant as legal advice.
There might be a few things you can do. First, the photo is protected by copyright law, and if you took the picture, you are the owner of the copyright. In that situation, you can send a DMCA takedown notice to internet website on which the photo appears. Most reputable sites will have information regarding their DMCA takedown procedures somewhere on the site (maybe under a link called "copyright" or "legal"), so you can dig around to see if you can find that. Sites are legally obligated to take down copyrighted materials if a proper DMCA takedown notice is sent by the copyright owner. If you didn't take the photo, but you know the person who did, that person could do the same thing. Here's one lawyer's explanation of the process: http://www.ipwatchdog.com/2009/07/06/sample-dmca-take-down-letter/id=4501/
Another possible option for you if the other person's behavior amounts to blackmail or extortion (i.e. she refuses to take it down, or threatens to distribute it more broadly, if you don't do something), is to contact the the Internet Crime Complaint Center (IC3), which is a multi-federal agency task force made up by the Federal Bureau of Investigation, the National White Collar Crime Center, and the Bureau of Justice Assistance. The website for the IC3 is http://www.ic3.gov. One of the things that the IC3 investigates is online extortion.
The information provided here is general in nature, is not legal advice, and does not create an attorney-client relationship with Will Montague or Montague Law PLLC.