These can be anything you want, I just named them posts so it makes sense in a blog format.
The info at the top of this file is called ‘front-matter’ and you can add anything you want up there.
In the templateKey field above you can see this post will use the post template.
You can put images wherever you want. In this setup I made an images folder for content file images specifically, and then another site-wide one in src/asssets/images for when you wanna add stuff like logos, etc to the site design.
You can add images by linking to them, you just put the alt text in the square brackets.
This is what makes the image below appear.

It does all the crazy html image processing behind the scenes, making it optimized for retina screens, etc. Super easy.