I am not an attorney. I am an artist. I don't know about Thailand or international law, but in the USA there is a "common law" copyright. You do not have to register or remind anyone, only prove that you completed the piece on a certain day. There are all kinds of little tricks people will tell you about. The best one is to have it notarized. That isn't really necessary, though, since no one is allowed to reproduce your work without your written permission. Still, to be sure, get documentation. If the work was done on a computer, it is automatically documented.
We can thank Edgar Allen Poe for this.
Rip offs are rampant these days. It would not be worth your while to chase down a theif from the other side of the world unless they make millions from your work. If some kook lifts your stuff to impress friends on Facebook. Send them a cease and desist order, or let it go.