A User Card is shown when clicking on a username or avatar in Discourse. It gives basic information that the person filled out in their profile. It is possible to upload a User Card Background which forms the background image of this pop-up.
Users also have a profile page dedicated to them. You can visit this page by clicking on the user’s avatar or username in their user card.
The profile page can have a separate image along the top. This image is the Profile Header. When visiting your own profile, make sure to click the 
Expand button to view your own profile header image.
You can see examples of these images by clicking on my username @mike or avatar next to this post and then clicking again to visit my profile page.
Both images can be set by visiting your profile page. Scroll down until you see the Profile Header and User Card Background headings. Upload appropriate images. 
And click the Save Changes button at the bottom when you are done.
@discobot fortune
Mike
Most likely