tickets are yet another form of authentication used by the internal AFCS code (basically a ticket is something that identify a user in a currently running room). It's similar to the external authentication tokens but tickets came way before eauth tokens, to separate generic user logged into our system from users actually accessing a room, that we are still using them.
userImageURL are URLs that you could use to assign an "avatar" to a user, but you are in charge to set them and use them in your own roster implementation, and currently the only "application" that uses them is ConnectNow.
Regarding upcoming REST API, it's still early. We have some ideas, but they are not fully baked yet. And then, who writes documentation before the implementation is done anyway ? :)