Sseguin As jordanjay29 mentioned, you can use Flagrow Upload to upload images.
However, without the extension, one can still link images.
If the forum has Markdown enabled, you can use Markdown to format your post.
If the forum has BBCode enabled, you can use that to format your post.
In markdown, you can link images like the following: 
With BBCode, it's done like this: [img]image url[/img]
However, I recommend searching on this forum before creating a new discussion if your question has been answered. In this case, it has been, many times.
See https://discuss.flarum.org/d/5550-how-to-embed-an-image-video-or-banner-to-my-post/2 for oan example.
