I found the answer to this. It's in the settings:
Admin > Settings > Uploads > Profile Covers > Default Cover Images
I've been waiting for days but i don't get any reply, i tried by googling my issue but same.
The problem is that members of my forum can't update their profile pictures or banner. They change it but nobody can't see the new picture until they clear caché. How to fix that?
It's likely that this is a bug, not something you can easily fix. Open an issue on GitHub.