Jermaine Cunningham, an NFL free agent, is expected to be sentenced to probation in June.
The 26-year-old Cunningham pleaded guilty in state Superior Court to invasion of privacy, illegal transport of a weapon and possession of hollow-point bullets.
The charges stemmed from a domestic dispute at a residence in Summit on Dec. 29. A loaded handgun and the bullets were found in Cunningham's car. Prosecutors say he posted a sexually explicit image of his estranged girlfriend on Instagram without her consent.
The decade-old New Jersey law being used to prosecute Cunningham was the first of its kind in the country. It forbids disclosing photos or videos without consent of someone "whose intimate parts are exposed" or who is engaged in a sexual act. Under the law, an invasion of privacy conviction is punishable by up to five years in prison.
Sixteen more states have passed similar laws, including 14 in the past two years. At least 10 additional legislatures are considering revenge porn laws.
Cunningham had sought to enter pre-trial diversion program that would have enabled him to have any conviction erased from his record, but a judge denied that request two weeks ago.
He legally owned the handgun but illegally transported it when he had his car sent to New Jersey last year, the Union County prosecutor's office said.
Cunningham was with the Jets last year. He spent the season on injured reserve after tearing his Achilles tendon in a preseason scrimmage.