Extremely late, but apparently the source for this image is an artbook titled Kenji Tsuruta Hita Hita (or as Anime News Network calls it Hita Hita: Tsuruta Kenji Illustrations Shū). It's an artbook featuring several examples of young attractive women in casual nudity/exhibitionism.
Ah wouldn't it be nice if the world were... damn it, who am I kidding? I can't help but ruin it for myself because cold, wind, and uncomfortable seats exist.