Resolved by updating NodeBB to the latest version.
Solved Random profile backgrounds?
I noticed here that the background images on profiles are randomly set. Looking at the plugin list I can't seem to figure out if this is done with a plugin or something else.
I'd love to set a random default photo for the users on my forum.
I found the answer to this. It's in the settings:
Admin > Settings > Uploads > Profile Covers > Default Cover Images