wumix That's what we're talking about as well.
That's not something Flarum supports right now. You need to use an external image host for the time being.
then once you have the image hosted the syntax for making the image is:
![alt text](https://some.host/and/path.jpg)
fragicide further suggested a way to integrate Imgur, an image hosting site with flarum in such a way to make this automatic.