The 3-D image was posted on a number of al-Qaida Web forums, the New York Daily News reported Monday. Paul Browne, a spokesman for the New York Police Department, said that on one Arabic-language site, gulfup.com, it was in the "art and design" section.
Browne said the poster is believed to be an Egyptian.
The FBI issued a statement saying its Joint Terrorism Task Force was investigating the image. The agency said there was no "specific or credible" threat against New York.
The image shows a man and a boy on a street with the man glancing over his shoulder where the words "Al Qaeda coming soon again in New York" are superimposed on the scene.