The earliest mention I can find is on this old website cached in 2003, metaeducation.com
[EDIT: the date was wrong. Cached in 2009]
http://74.125.95.132/search?q=cache:5IjJAdzEeNkJ:metaeducation.com/%22thislooks+shopped%22&cd=4&hl=en&ct=clnk&gl=us
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.
http://www.fark.com/cgi/comments.pl?IDLink=1423724&hl=Charlotte-Churchs-topless-pics-stolen-The-Sun-is-there-but-with-no-pic-goodness
[EDIT: It says "This looks shopped" but says nothing about "seen plenty of shops in my time"