1. You can either just serve an image on a URL and the device will display it, refreshing whenever the image changes:
https://www.invisible-computers.com/invisible-calendar/image...
2. Or you go one step further and wrap it into an API with a settings page, which will also allow others to install and use your app:
https://github.com/Invisible-Computers/image-gallery/blob/ma...
To be fully transparent: I know that many people are using the first approach to display their own designs, but I haven't had much uptake yet on the second option to build a public, installable app. So if anyone is interested in trying this out, please contact me! I am willing to cooperate closely and iterate on the API where necessary.
(I hope that this is sufficiently relevant, even though I am tooting my own horn here.)