On GitHub you can, using a browser, immediately start editing text files, and so write markdown.
You can embed images into markdown:
Markdown is a simple way to turn plain text into formatted text, much like HTML, but with a much more lightweight syntax.
- I tend to use Visual Studio Code for editing, as it support markdown out of the box. Pressing Ctrl-K and the V opens a live preview.
- A nice on-line editor is JBT markdown-editor which works in any web-browser. You can't preview images, of course.