Visit popular online Japanese goods and anime shop JBOX and take a look at the buttons on the left hand side.
Take a look at the button called Calendars.
Could it really be that image?
I am honored and embarrassed in equal measure, to be sharing the sidebar with Kagami Yoshimizu and Domo-kun.
Are you going to e-mail Peter and ask where he got it? Maybe you made it to 4chan. Or some unexpected google query brings you on top (try “anime calendar” in Google images).
I’m guessing he got it from Danbooru. There’s a “sixten” tag on there, as well as J-List ads.
Did Jason(ko) find out? S/He’d probably be interested to know…