Unfortunately I couldn't find a proper answer, so I have spent some hours of trial and error, here are my results:
- Use RSS, it seems to be more widely used than Atom
- By adding the xmlns:content="http://purl.org/rss/1.0/modules/content/" Namespace, you can make use of the content:encoded tag which is recognized by Google Newsstand
- Google seems to make use of further images on your blog, still not sure about OG or schema.org
- Use an image tag at the beginning of the content:encoded blog, it will be the title image in the App, the OG-Dimensions (1200x630) seem to work just fine
For further details, please look at the example in blog post: http://www.webmastersdiary.de/blog/google-play-kiosk-newsstand-rss-feed-spezifikationen/
(It's written in german, but you should be able to get the XML example ;-))