This site contains text descriptions of spoof magic cards, but doesn’t even have any photoshopped images of them.
It started in 1994.
It seems that spoofs of magic cards, and even physically real counterfeit cards were a part of the Magic: The Gathering fandom already only a year after its release. I don’t know that we’ll ever find THE FIRST EXAMPLE of this, as it likely occurred in multiple places simultaneously.
So far it looks to go like this:
Early text descriptions of spoof cards in 1994
There may have been some early photoshops between 1994-2000 but I have yet to find any.
Most search results that return images seem to come from 2001.