How so?
External images can be added using bbcode or markdown.
You need an extension if you want to enable file upload directly on the forum. I don't think FreeFlarum allows you to host images on the forum itself though, but you might still have the option of using Imgur.
If the problem is specific to FreeFlarum, you should contact their support. FreeFlarum is run by community members and isn't affiliated with Flarum.
