How to add a preview image to a content type?
No, this functionality is not in Drupal core.

It could (at least in principle) be added through a contributed module, but AFAIK, there is currently no contributed module offering this.

It may not be trivial to add this feature in a meaningful way.  It is not unusual for Drupal sites to have content types that uses the same visual layout - but different workflows and access rules - for different use cases.  A "preview image" would not distinguish them from each other in such cases.

While the idea of different content types for different purposes is at the core of the "Drupal way" of building a site, I've noted that many ordinary users, editors and even admins struggle with this particular idea.  The problem you mention: "the editor who doesn't know which content type to choose for his content" is a real one.

I've not found a really good solution to this.  In the real world helpful content type descriptions are often not read by users/editors/admins.

My solution so far is to offer a very restricted number of content types to end users, and use very descriptive names for the types they're exposed to (e.g. "Personal blog post", "Embedded video").