The moment that your art is created you have an instantaneous copyright. There are a number of ways to protect yourself from theft or piracy.
How To Protect Your Art From Theft or Piracy
1) Sign your art work. Be sure to wait until you are done with it before you sign it. You can sign the back of it but if you are going to display it online you almost certainly should sign the front.
2) You can register your creation with the Library of Congress. Go to the copyright office of the Library of Congress website. You’ll have to pay a fee but you can register your piece there.
3) There is the very popular watermark approach. Display your work with a watermark over it.
4) Display your work as a slideshow since “cut and paste” will not be possible with this approach.
My personal opinion is that you should sign all of your work, even if it is very subtle. List it on your website or artsoko.com with the publication date. Also, if it is within your budget copyright with the Library of Congress.