I was trying to speak in laymen terms, as in Ren'Py's graphics there are parts which serve as backgrounds or whole pictures, they are called "scenes"; and there are parts called "characters", placed on top of scenes. "Characters" have transparent parts, so png's are at least somehow justified for them; though nowadays it's better to use webp files: they can have transparent parts too, and they can have small size with decent quality.
That picture was a "scene", so I would call it "background", even if there are no "foreground" layers. Using png for scenes is not justified. That's just bloating the game files size. And look at the speckles... Actually in my screenshot they are softened by conversion to jpg; in the original they are even worse.