nevermind-studio hey, this is technically possible, but it would require a few changes: implementing OAuth authentication with your Imgur account, managing tokens and refreshing them, moving the upload call to the server to avoid leaking tokens.
Unfortunately I'm not that involved into Flarum stuff anymore, I'm committed to maintaining this extension since I know it's helpful to many but adding this feature would mean rewriting a good part of it and I'm unable to do this personally, I'm afraid.
I understand this might be a problem though, if anyone wants to take over the development of the extension and/or fork it, feel free to do that!
Sorry for not having a better answer 😅