My iOS app im building used various images (.PNG files, some are via the Assets Catalog). For now these images are part of the app. In the future more .PNG images will be downloaded as part of an In-App Purchased package (.PKG).
Do each .PNG file need to be registered into the Document Types list in the info.plist?
No. PNG is already a standard registered type. And registering Document Types is for registering a type, not a list of files using that type.