A wizard did it is a phrase from a picture talking about god.
If you want to get technical, "God = Wizard" has been around much longer than that particular internet meme. My guess is that the person that made that particular inspirational poster did so after seeing an old copy of a reader's digest or something similar with humor cartoons in relating the same thing. That person then thought to himself: "won't I be clever if it copy this and post it on the Internets!" thinking that no one on the Internets has even seen paper, much less an old magazine, and therefor no one can then reply to his poster with the phrase: "old! GTFO noob."
I think I have an old farside book around here that has one that literally says "a wizard did it" but I can't find it.