Flarum (the software) supports uploading images via a third party extension:
https://discuss.flarum.org/d/4154-flagrow-upload-the-intelligent-file-attachment-extension/443
discuss.flarum.org (this website) doesn't use this extension, so you cannot upload images here. You can however embed images that are available online via markdown:
or BBCode:
[img]https://discuss.flarum.org/assets/logo-2q2rcevl.png[/img]
both resulting in:
