My guess, you have to host it online first, then reference that link.
Haven’t posted pics here myself but that is how I’d imagine it to work.
If you can’t post it here, don’t worry. Just open a free account at Coroflot or another portfolio site and upload your work.
However, if you are serious about design, a website will be very helpful if not essential, especially considering that you are interested in UI/UX work.
Server space is a small investment but highly valuable in the long run.