Most Fediverse software supports image uploads, so there’s no reason to use Imgur for image hosting. Hell, even on my small single-user server (atomicpoet.org), image hosting is easy peasy. Not only is Imgur not needed, they’re an annoyance for those of us who are used to seeing images natively on the Fediverse.
I know habits are hard to break, but just remember: this isn’t Reddit 🙂
Depend on how it’s setup. Some servers are setup to cache images, some don’t. Mine for example doesn’t and streams directly from source instead so i don’t waste storage.