I’ve had my work ‘stolen’ (that is to say: my copyright breached) numerous times. A good example of a copyright breach is having your original artwork printed on apparel and sold for profit without your permission or knowledge. (Note: art theft does not include ‘style borrowing’ or ‘style theft’ – styles are not copyrighted!) Fortunately, with help from my fanbase, I’ve usually managed to have such items removed from circulation without having to take legal action. I’ve also sent out many DMCA takedown notices, and sent out cease-and-desist letters, which helped to remove the stolen work. People have asked me what can be done to prevent such things from happening. My personal opinion is that there is no solution that completely guarantees that your art will never be stolen, except for one thing: Never showing your art to anyone. This has the disadvantage of your art never gaining positive exposure of any kind, so I don’t recommend it. Any preventative measures you take should be weighed against the greater benefits you may receive from sharing your work. For example, I chose to avoid using large watermarks on my artwork because I find that disruptive. For every art thief who no longer has to bother with photoshopping out the watermark, there are numerous other people who enjoy the experience of seeing my work as it was intended and share it with others, which in the long term brings me exposure, clients and income.
One important thing you can do is avoid sharing high resolution versions of your work in any way, shape or form unless it is with a trusted client or business partner. If you find your copyright breached, contact the people who are doing it and inform them of the breach in a professional and straightforward manner. If they are not willing to remove the item or discuss the problem, contact someone with legal experience and, if you can, join forces with a lawyer, who can assist you in sending a case-and-desist letter.