"As perfect as a picture", ie. it doesn't change. Unless you are Dorian Gray. So we use "picture perfect" do describe a scene that is complete. If you painted or photographed the scene, it would look perfect.

Picture perfect means that something is is exceptionally or especially perfect. Really perfect. If you were walking in nature and you saw a view of the mountains that was really beautiful, you could call it a picture perfect scene. It's so perfect it could be a picture or worth a photograph. Picture perfect memory would be a really good memory. Someone can look at something and remember all of the details, as if s/he were looking at a picture.