dont put it on the web? noone can take it then, its a risk you take when you upload to any site. generally if you upload smaller resolutions its less tempting to reproduce because they wont have the quality when reproduced
Digimarc it! For a fee (one needs to register), and using Photoshop, you can embed within the image, a digital "watermark" to protect it from unauthorized reproduction.
When you upload to Caedes, the maximum resolution is 1600x1200, so as long as you have a copy at a higher resolution, you can prove ownership. Digimark is a good way to do it too.