The only secure solution to really protect your photos is not to post them on the internet at all. That however is not the best solution if you want to show your work to the world which can lead to potential sales. From recent experience, there is always going to be some idiot who thinks they are above the law when it comes to copyright infringement and image theft. Most countries protect your works automatically when they are created and when it becomes available on the internet. It is very important to always specify ownership of your images and to heavily watermark them with copyright, logos, etc.
Never upload the full resolutions of your photos on the internet. Always use a much smaller resolution of the original and stamp it heavily with a watermark. It is best to use a clear watermark where others can at least still view the image. No amount of coding for your website will prevent thieves from stealing your photos. Once it is posted it can be stolen. There are methods such as disable right click, print screen and so forth to deter image theft but it cannot be totally stopped. Low versions of your photos that are heavily watermarked is the best legal statement that the image belongs to you.
Another alternative is to sell your images on CD or DVD. This way you could have a registered owner with a unique key. However, there are still drawbacks to this because cds and dvds can be shared easily with others and ripped. This method could also be a slow down in sales because most people want their product for download immediately once they pay for it. This could be perhaps used as a backup selling source with a registered owners list so you know who has a copy of you photo disc. This would make it much easier to track illegal usage and distribution of your pictures.