Yesterday, I decided to try to track it down. It didn't show up in Ohnorobot, so I had to look for it manually. I figured it was about 5 years old, so starting 1st January 2013 I looked through every SMBC cartoon in order of appearance. Several hours later, when I got to 2017, I accepted that I'd missed it. I went back to 1st January 2011 and started searching from there.
Needless to say, another hour or two later I finally found it on 14th December 2012 — a mere 19 days before I began my initial search.
This is the particular panel I wanted it for, to use in one of my lectures.
I might see about licensing it for the cover of my How to be a God book (if I ever get to finish it), too.