User Pictures

Had a user leave a comment
“How can I add a picture on the side of my posts!?”

Now, from what I’m gathering when i add someone to the blog I send them there invite and sometimes the new users have a picture next to there name, I take it that’s because they have an account with
https://wordpress.com/ ???
If, so okay.

But, how can I have my other 3k users who don’t have an account with
https://wordpress.com/ have pictures cause I’d like to say a lot of the users are just blank picture, https://www.gravatar.com/avatar/ad516503a11cd5ca435acc9bb6523536?s=48

Well something like that, now can users go to there profile and do something about this add there own picture or something

Thanks
-Dave