Can a user be banned on their host instance for breaking the host's rules on other instances?
Can a user be banned on their host instance for breaking the host's rules on other instances?
Hell yeah.
What ration we talking about here?
That would be up to the admin of your host instance. Hypothetically they can ban you for any, or even no reason. There are no software or rules preventing it.
So I would suggest making an account on the instance you want to post these photos to.