Yeah, that's top 5, you're asking for top 15. The URL that is available for the avatar via the Steam Web API is over 100 characters (
this one is 117 characters). So, you'd be hard pressed to even get 15 avatars in an MOTD and that wouldn't include anything else i.e. just images.
I can't download those files so I can't see if they've implemented any special functionality that I don't know about but I suspect there isn't any magic feature that would make it work for this many.