The Press Complaints Commission said the photograph, which was included in a gallery of mobile phone pictures, was a "serious intrusion" into the girl's privacy and had a "significant effect" on her emotionally and academically.
The commission also complained the magazine failed to respond to the charges in a timely manner.
The girl’s parents went to the PCC after the picture was published in the magazine's April 2007 issue.
They said the photo was taken in 2005 when their daughter was 14.
It was submitted to the magazine by someone with whom the girl had a cohabiting relationship.
FHM said it receives about 1,200 photos of women either topless or scantily clad for publication each week, adding it was "extremely surprised" to learn the girl was 14 "as she certainly appeared to be older," the PCC reported in its ruling.
The magazine has taken steps to ensure the situation never occurs again, has promised not to republish or syndicate the photo and even offered to write a private letter of apology to the complainant.