There's various tricks you can use to slow people down but ultimately no you can't. For example you can set the image as a background image in a div and then use a transparent .gif to cover it. When the person goes to save the image through right click they save the .gif These tricks will only prevent the average person who is not determined.
A watermark helps but the issue arises that light ones can be edited out even by someone with minor skills and heavy ones really don't look that good. My advice for a watermark if you're going to use one is a gradient fill on a texture fill so it's not consistent. This will prevent someone from easily removing it if not making it impossible.
Be sure to fill in the EXIF/IPTC data but that can be edited out too.
Last but not least.... "Steganography". It won't protect the image from being copied but you could certainly prove it's yours beyond a doubt assuming it's not re-sampled to the point the invisible watermark is destroyed.