It was done by an LA deejay and actor named George Mahlberg sometime in the mid-90s. He photoshopped himself into the picture, behind the "keyboardist."
I read an interview with him once where he said he was looking at the picture of Oswald being shot, and thought he had a "blues face." And so "In a-Gadda-da-Oswald," the greatest Photoshop ever, was born.
Indeed. I used to share it on Facebook every anniversary of JFK’s death. Few of my friends saw the humor in it that I did.