The code mirror makes good job of previewing things like headings.
Could it preview images?
I'm mostly all for ditching "preview" altogether.
Good question! The editor can technically handle image previews, but not in Joplin. The next item on the Joplin roadmap is support for plugins. Once that's done I hope to make a plugin that will add this (or maybe someone else will beat me too it).
Didn't know that plugins are next todo. I thought that it will take a while until support for them will be introduced.