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I have a development app using the developer server and everything works fine except the screenshot and logo defined in the manifest.app in my module. Those images don't come up. That is bc a url starting with '/' (local) is being passed to apps_retrieve_app_image(). Now first I am not sure why we are doing http requests to fetch and save those images locally. Personally I think you could just link the src of the image to the remote image path.
That being said I added a small patch where I force the url to be absolute. That fixes the issue for me, but that is just a suggestion. I am attaching this diff for reference