I am not a lawyer. [ Mandatory disclaimer. ]
In general, in most countries, there is no need to take any specific action to register a copyright.
It's helpful (maybe essential in some situations -- you need to ask a lawyer) to put the copyright symbol, the word "Copyright", your name and the year.
If you have clients your contracts with them should be explicit about who owns the copyright to work you do for them or license/sell to them, and what they are permitted to do with your images.
The real issue though is what are you going to do if someone steals one or more of your images? My response would be somewhere between asking the person to stop (maybe they didn't realise the image was copyright) or just shrugging and ignoring it. There's no way that I'd go to court, and particularly no way I would go to court in a foreign country.
If your images mean lots of money to you, consult a lawyer. If not, just relax. Most people don't steal your work (the good news) and you can't afford to do anything about it if they do (the bad news).
All IMHO, of course.
P.S. An exception might be if an image or mine was used in an advertising campaign. Then it might be worth suing to try to get paid, but it could be difficult -- still costly and with an uncertain outcome.