This doesn't help Iñaki, it's as easy to stole a picture like making a capture of it, any of our propgrams like PSP, etc... has this option, even with the right click of your mouse you can save a picture to your computer.
I really think that the only way to protect them is with a watermark or uploading them in a smaller size but this second option even doesn't help alot because there are splendid programs to change the resolution of a captured image.  ;(