Those are exactly what Igiss means by "external host" because, well, they aren't connected with this site in anyway whatsoever, and, apart from a tiny bit of text, they don't even use the forum's server to download and show up.
I usually do use external... Because I don't like those little thumbnails. That, and I got used to doing it while Firefox had that small bug where images from thumbnails show up as a huge amount of random characters and boxes... x.x