here is an example http://www.sohtanaka.com/web-design/protect-your-images-with-css-watermarks/ but this useless as someone who know how to use "inspect element" in chrome will easily save your image without watermark.. in this example you only cover your image with a gif that will be saved if a user cliks "save image as".
So I am thinking that you need more like a image-generator that will cover your uploaded image with watermarks. I could be wrong, if anyone knows a CSS way of doing it, go ahead.