You can include the fields you want for the view. For example change: https://github.com/MaddinXx/ch.schwingenonline.android/blob/develop/assets/www/routing.json#L11 to get_recent_posts/?include=id,title,excerpt,thumbnail and include it in the view:
https://github.com/MaddinXx/ch.schwingenonline.android/blob/develop/assets/www/tpl/news.mustache (you will have a new placeholder {{ thumbnail }} there.
You can see all available fields by calling the URLs WITHOUT the include parameter (and ?dev=1)