If you're applying surface ornamentation to a shoe, (imagery like this, that is considered separable from the footwear) your best protection is probably copyright law because it's automatic and because you can stop others from reproducing the image on anything, not just shoes. You should register the work if you're concerned about it being ripped off because copyright registration provides some advantages in a lawsuit. You can register all your designs as a group but if one of your designs is more popular than the others, you should probably register it separately. You can find help online, and if you need more assistance, check out our book, The Craft Artists' Legal Guide.
Protecting the shoe's design. If you want to stop the specific use of your image in connection with shoes, (or for some reason you want to protect the design of the shoe itself -- the shape, proportions and "sculptural" elements), you might consider filing a design patent. Design patents protect shape, design and ornamentation for useful objects. Companies with large design patent portfolios, such as Nike, successfully use design patents to stop others from making similar shoe designs. We discussed the differences between copyright and design patents in a recent post. The big difference is that design patents are more expensive and take longer to acquire.
PS Good news for those who can't find what they want at Zappos: design your own shoes.