Tricky to find an exact photograph for this. On this Mo-an guide map (source) this is located along the path just above the gray building, which sources indicate is Ryouunji 「凌雲寺」. This photo (source) from further back shows the building with the path and low wall on the right. Here's a reverse angle look including the mailbox and the post that the girl is leaning on.
As per the map, the building in the background is Mo-an's 「田舎席」 , one of two historic chashitsu (Japanese teahouse) that still exist of the original eight built here during the Taisho era. Here's a photo (source) from roughly the same angle. This is from further off to the left.
