Never mind, found it digging the source. If someone is interested: you have to add "icon: <url>" to zapstore.yaml

Ideally the icon should be correctly extracted from the APK. Just FYI, this appears to be an edge case as we parse over 3000 apps and very few have issues with icons. Overriding in metadata should work, what do you mean it does not let you pick an icon? If you send more info I can help