How could this IMPOSSIBLE painting have been made?

Here is a photo of Elwood P. Dowd, admiring a painting of himself and his very good friend, Harvey the Pooka.

Very simply, my question is this: If Harvey can be seen ONLY by Elwood P. Dowd, then HOW was the artist who painted the portrait, able to see an INVISIBLE RABBIT, and then include him in the portrait? One cannot paint something that one cannot see, can one?

I’m getting a headache thinking about this. I want my Mommy!