I think its from some Ferrari film as a "dream sequence" is as much as I can find out.
Senna was in talks with Ferrari through most of his career (as are many drivers - everyone talks to everyone in F1..doesn't always lead anywhere).
Luca has claimed in the past that Senna had signed for 1995.
So yes, I guess its "fake".
Not to mention its almost impossible that Senna drove a Ferrari in 1991 even in a private test because Prost would have left immediately. Perhaps near the end of the season when Prost was kicked out anyway, but its very unlikely. It would have been big news and would be much widely reported, even if only 1 random photo got out.