How do I make a secure copyright image?

I only can offer a perspective under US law. Three things you can do to help you enforce your copyright interest in a phone.

Copyright in Vietnam 

Register the photo with the US copyright office. You now can do it online.

Put a copyright notice on it. For example, (c) 2018 Your Name, or “Copyright 2018 Your Name.” You are not required to register with the US Copyright Office to use the notice.

Put a watermark on it. Best approach, a watermark with your copyright notice.

Keep in mind that these steps will not provide 100% full protection against infringement. But these steps will put others on notice that you assert a copyright interest in your photos, and you will be in a stronger position in the future to protect your rights.