when the app goes offline, it should display some text or built-in image

The current behavior shows that the app is aware of when it loses connectivity. It displays a small green notification (should be orange or red-ish, incidentally, since it's bad news).

But it should do a little more, in my view. I do notice that some views (notably if the main url is not accessible) just display an empty page with nothing on them. I think there should be an image or some text that could be configurable in the app for the app developer to add, as a default.