how to add a Gravatar

I’m having trouble finding the spot to add a gravatar. I have my gravatar for my own user name, but the site that I am maintaining for a client has its own user name but I don’t see the place to add a gravatar. When a comment is made I see the little place holder where the pic should be but when I go to edit there is no where on that page to add picture.
I’m sure that every user name can have its own gravatar, right?
Can anyone help me with this?
Thanks, Kaycee7