It is from one of the endings added in the final release of the game. I think it is from the one you get if you don't get Chiho pregnant during the onsen.
Most people wouldn't know where most of the pictures came from unless they were following the developer so you not knowing isn't weird.
But isn't
You assuming the pictures were snatched from assorted Doujinshi?
it's in the NTRAholic, after you finish the game, you unlock the final power to see 4 Another story of Shota, that pic above you ask is the After story 2, then i bet you never see this one too.
