Profile Images

The field is empty!
Do I have to insert the image through the image editor?   

It's a regular profile field. You supply an canvas image from CB profile edit. You don't seam to be familiar with CB. I suggest reading through the CB Premier Book to get a grasp on the basics.

www.joomlapolis.com/downloads/documents

You can also take a look at our demo site below to compare configurations if you need additional references.

demo.cbdemosites.com/

Below for example is a working grid userlist with canvas image displaying fine.

demo.cbdemosites.com/demos/community-builder/user-lists/grid-layout

I found the reason why the canvas image is not loading!
I changed the domain and moved to another one, but the profile background image url is still from the old domain!
How do I insert a new default image for all profiles, from the new domain?

I see : #cbUserTable .card-header {
    background: url(https://oldurl/images/backgroundcard.png)!important;

Where do I find this css line, to modify it? 

I changed the domain and moved to another one, but the profile background image url is still from the old domain!

You need to fix the live_site in configuration.php then. Either set it back to default empty string or set it to your new domain. We don't hardcode domains anywhere within CB.

Where do I find this css line, to modify it? 

That isn't CSS provided by CB. So that's CSS you've added. You need to change it wherever you added it. Check if you added it to CBs CSS Overrides functionality shown below.

www.joomlapolis.com/blog/kyle/18711-template-css-overrides-made-easy