I've never done watermarking, since it's useless. There are so many different ways to save an image. Print Screen, using a screen capture thingy, using a browser that won't display watermarking, etc etc. When I looked into it a while back it seemed to be too much trouble for too little reward.
How is there any difference? A pirated version is just the retail version with a tiny bit of modification to run sans disc... the software doesn't know its pirated.
Pirate Copies are great trust me though the disk is a dead give away so use em in secret and don't try the water something I tried it and ya don't want to and before I'm covereed in reporters I do use pirate copies but I can just say I'm 11 and don't know better...hehehehehehehehehehe (cough cough got to stop that laugh).
There's anti-hotlinking Javascript out there somewhere, however the ability to apply this outwith a webpage and onto a single image is unknown . . . and there's always the old screencap option.
Best way would be to put a graphical watermark, or a signature at the bottom of your sigs in an area which when editted out would be noticable.