Moss chooses on your behalf the latest official images for your operating system on each cloud provider where you launch a server.
Canonical publishes Ubuntu images for Amazon in both Community and Marketplace versions. Moss prefers Community images over those from the Marketplace because the former ones don't require any action on the user's side.
On the other hand, CentOS only publishes their Amazon images on the AWS Marketplace. Despite image usage is free, Amazon requires an explicit opt-in from the user in order to employ such images. Hence, in order to launch CentOS servers on Amazon with Moss, the user must opt-in from the account that will use to create Amazon credentials. Note that at this moment CentOS support has been disabled in Moss.