I couldn't tell you exactly, but I'd bet a cookie it came from someone in the Fark, Something Awful, or 4Chan forums. It's amazing how many hilarious memes and Pshops those websites have spawned.
After a little googling, I found multiple sources that claim it came from Something Awful's Photoshop Phriday. I'm not 100% willing to accept that, since Fark users and Something Awful users routinely steal **** from each other and claim it for their own.
Users from both sites steal memes from 4Chan, but nobody likes to admit that they actually frequent 4Chan. And that is completely understandable.