Child Pornography
Child pornography is a form of child sexual exploitation. Federal law defines child pornography as any visual depiction of sexually explicit conduct involving a minor (persons under 18 years of age). Images of child pornography are also referred to as child sexual abuse images.
Federal law prohibits the production, distribution, importation, reception, or possession of any image of child pornography.
From http://www.justice.gov/criminal/ceos/subjectareas/childporn.html
Nude photos of children at a nude beach is legal in the US