I use Joplin for note taking, as an alternative to notion, it great so far but I am wondering if I can show images in the editor window directly instead of just its text. e.g.
If you have other tricks for editor window like coloring lines and other useful things for note taking that would be good, and if Joplin can read custom formatting that is not supported in md standards that would be good also.
Joplin actually has two different editors (or technically unlimited seeing as you can open any note in whatever editor you like).
The markdown editor (which is what I assume you are referring to) is the one that is designed for entering markdown formatted text directly. As default this shows only the raw markdown with some mild syntax highlighting and font style changes, the rendered output of this (including images) is on the markdown viewer (view > toggle editor layout) or use the button in the top right on the screen
You can actually add more functionality to this editor by installing the "rich markdown" plugin which, amongst other things, adds inline images to the markdown editor directly
The richtext editor is designed more like a traditional style editor where you do not have to insert markdown tags manually (although in the background this will still be converted into markdown when the note is saved).
This editor has a number of limitations you have to be aware of: About the Rich Text editor | Joplin
You can access it by pressing this button in the top right.
It too will render images 'inline' with the text.
As for custom formatting, there are a number of markdown extensions, some enabled and some that can be added via preferences/options > markdown.
A full list (including the extensions) of what it supports can be found here - Markdown Guide | Joplin
Other custom syntax and functionality can be added by various plugins.
That command isn't part of standard Joplin, it is provided by the rich markdown plugin and is also activated by just turning on the option within the rich markdown plugin settings Render images below their markdown source