How to reuse existing images in field type "Image" I have custom ContentType with one field of type Image.
Currently, when I edit a node I can only upload new images from local to server.
I want to have the option to select images already existing on the server.

I think that the way to go is to use the EntityBrowser and/or FileBrowser modules
but I don't know how to make this work.
I can create the EntityBrowser instance but how do I tell the ContentType field Image to use it?

I'm on Drupal 8.

Your help is appreciated.
--Josef
Projects are built from the start to use the Media module and reference entities instead of files directly. That is how Entity Browser can help. 

Here is how it's usually done:
https://www.drupal.org/docs/8/modules/entity-browser/how-to-configure-an-image-widget-to-re-use-existing-files.  

More useful resouces:
https://www.drupal.org/docs/8/core/modules/media/faq-transition-from-media-entity-to-media-in-core
Is there any way to reuse the same file or image?

There's a way to convert your files into entities, there is the Migrate File Entities to Media Entities module, and some useful info about it:
Migrate File Entities to Media Entities in Drupal 8,
How do I migrate file entities into media entities?