[pietroalbini]

AFAIK they must certify their images, which means they should offer a stock or a stock-like image approved by Canonical.

Unfortunately the only source we currently have is that tweet, which isn't the clearest one.

[icebraining]

Ubuntu's IP policies are public, and you're right, modified versions must be approved by Canonical: http://www.ubuntu.com/legal/terms-and-policies/intellectual-...