When you submit a plugin to WordPress.org, you need to put everything in a zip file that you must put on your server. After validate it, you receive a mail with the new svn link for the validated plugin.
Readme.txt link all images to à folder called assets where banner, screenshots, logo take place.
For the validation, you can reproduce the folder structure in your zip, as it will be in the svn :
Svn plugin folder
----- assets
--- banner-772-250.png
--- screenshot-001.png
--- icon-128x128.png
----- trunk
--- readme.txt
--- plugin_main.php
--- includes
.....
----- tags
--- 1.0
When online, visitors will see the readme.txt from the trunk folder, the header line ‘Stable tag’ will redirect user to right current version (tags/1.0 or trunk) and generate the zip file for the download.
You can embed video from youtube with the markdown [youtube https://youtu.be…..].
An image markdown for image look like this for me:
1. The screenshot description corresponds to screenshot-1.(png|jpg|jpeg|gif).
What you will do, will always be under control, try and you’ll be able to remove, modify everything on your svn directly.
You will find on the web, a lot readme.txt validator website. I use generatewp.com to create it quickly.
You can read the WordPress documentation about readme.txt : How the readme.txt works, the page explain how the file is parsed and a brief explain of the markdown.
You can also read the developper Plugin Handbook, the WordPress.org part, talks about svn, readme.txt, asset and update Plugin Handbook WordPress.org
Hope it helps.