Go to the tab General => Navigation, drag the green square on the right with Custom Route next to it over to the navbar looking area near the top. Then click the new Icon that appears and fill out the fields, put the url in the route field. Then click Enable and then click the Save Icon at the bottom right.
Grabbing avatars from an external URL
I'd like to know if there exists a method to alter the way how avatars are automatically assigned to users (like Gravatar). For example, let's say I have an user named Foo and his avatar is located on a remote server & domain, on the URL domain.com/avatar/Foo.png. Basically, all I need is that NodeBB would automatically take the avatar from a specified URL and assign it as the user's avatar when he/she registers for the first time.
has been answered recently here
@markkus Extending change picture inside profile to allow a URL would be a nice addition for this.