You are able to save posts made by any accounts you have added to Flume to your Mac, from the Post macOS Menu Bar menu, or via the contextual-menu.
To respect the copyright of the author, you cannot save posts that you did not post yourself. You can however, repost items and ensure credit is provided to the original author. Note that you cannot repost items if the author's profile is set to private.
If you believe someone has infringed your copyright on Instagram, or to read more about your rights, see Instagram's Help Center.