Sorry for the delay on this - thanks for sticking with us.
Regarding graphical materials for the marketplace itself, we are preparing guidelines on our end right now and will publish them as soon as they are ready. One thing you can do for the time being is to upload any images you would like to the app on your account before publishing, as these can be switched around and replaced as needed.
As for asset hosting - any current host you may be using for hosting static files (such as CloudFlare) can be used for hosting these images/videos.