The New York Police Department and the FBI discovered the graphic on the internet on Monday.
It features a photo of the city at sunset with 'Al Qaeda' in bold type followed by 'Coming Soon Again in New York'.
Both the FBI and the NYPD said investigators have found no evidence so far that the posting represents a credible threat against New Yorkers.
The graphic was seen on several Arabic-language websites, including gulfup.com, according to NYPD spokesman Paul Browne.
Browne said is was posted in the 'artwork and design' section.
It has since spread to numerous extremist forums.
The NYPD wrote in a press alert its Intelligence Division's cyber unit is investigating the origin and significance of the graphic.
'Until more is learned about their origin, we take all threats against New York City seriously,' read the statement. Read More