There is no form of protection that will protect your designs 100% of the time if the images are online. Every form of protection can be defeated using one method or another. Registering each design can get expensive so that's usually not an option unless your designs are very valuable.
If you have your designs displayed online then you can add watermarks to your designs, disable the right-click button on a visitors mouse so they can't easily save/steal the image and some other forms of protection. However, like I said above, there are no methods that will protect your designs 100% of the time.
You can use a service like Digimarc (or similar service) that uses digital watermarks that are embedded into the image digitally and that's probably one of the best ways to protect your designs. They have some other features as well to help protect your images as well. I have never used them myself, I just did some research on them and other forms of protection for my images last year. It's supposed to be one of the best ways to help protect your work, but if you are thinking of this form of protection then you might want to check pricing because I can't remember if it was expensive or not.
