How to add an image to nodes?
My guess was to turn the image content type into a block, and if so, whats the best module to do that?

Bean

Bean is an acronym that stands for Block Entities Aren't Nodes.
This is a great introductory article about what the bean module.
What is Bean? Think of a Bean as a method to provide new types
(compared to node this would be a content type) which then provides an
add content interface to create as many blocks as you require (see
screenshot below). The bean content can then be placed around the site
just like any other block.
Beans were driven by the API first. This means that you can create
block types (bean types) all in a plugin class turning off the UI. The
entire configuration in code. No worry about feature reverts.

Here is a video tutorial
Basically it lets you add fields to blocks. This module comes with Drupal 8 core by default.