This question has been asked many times on the web.
I can answer it congruently - yes it was, and I was the doctor.
However, it wasn't doctored in the way that people think it was.
Here is the original anti-stress turtle photograph, taken by Alex Kent in Thailand:
Original Stress Turtle Image - fresh from the camera
And here's the "doctored" stress turtle for the StressFish article:
Do the anti-stress exercise with the turtle here: http://stressfish.com/stress-turtle.htm