nvhung Do you mean using the API and making a POST request to /api/posts or something like that?
If you mean using the editor, Flarum uses Markdown (not all of it) and BBCode to format one's posts.
To attach an image, you can use two ways.
If you want to use markdown:
![ALT TEXT](IMAGE URL)
If, instead, you want to use BBCode:
[img alt=ALT TEXT]URL[/img]
Result:
?