The earliest mention I can find is on this old website cached in 2003, metaeducation.com
[EDIT: the date was wrong. Cached in 2009]
But the explanation offered on the site implies that this was already an established meme, so it starts even further back.
The earliest appearance on Fark that I can find is in 2005.
[EDIT: It says “This looks shopped” but says nothing about “seen plenty of shops in my time”