The picture, posted on an aviation blog, was reportedly taken by a flight attendant to illustrate to airline managers the difficulty of dealing with passengers who cannot fit into seats.
The image, apparently taken on a Boeing 757, was sent to writer Kieran Daly and posted on his blog at aviation news website, Flightglobal.
It is not clear if the image has been altered but Mr Daly wrote: “This is sent to me with the absolute assurance that it's a genuine picture taken by a flight attendant at American Airlines. The F/A took it to show her manager what was happening on the aircraft (757???) and why she was unhappy about it. Seems the guy paid for only one seat and the gate staff let him board.”