Bandwidth is a two fold consideration. There is the site’s BW and YOUR BW.
The site BW is responsible for pushing all the data stored locally to you, and everyone else using the site. This has to be done simultaneously. Well, technically, there is generally no such thing as true multi-tasking and simultaneous downloads, instead the CPU opens different sockets, and then feeds one for a few milliseconds, then the next, then the next. It does so at such a great rate of speed, it appears to be multitasking, but at the millisecond time level, you can see it is only doing one at a time. Some multi-processors rigs can achieve true multitasking, but on the whole, it is serially accomplished.
Anyway, the more users logged on, the more NCT BW is used. Then, the more content stored locally at NCT, the more BW NCT has to push down to the user. If you store your pics off site, then THAT server will have to push the pic to you thus saving NCT BW.
On the user end, the pic appears in the NCT post, but actually comes from somewhere else. However, the user still has to download all of the data no matter where it comes from, and that affects user BW.