Well, do you think the type of person who was to snitch and publish your photo without asking will look at a "please don't use this image" tagline?
I started watermarking my images for the same reason you want this. I'd rather have people know where the image came from. Of course someone could always crop the watermark out, but that's the risk you run when you publish photos on the internet.
I'll see what I can come up with, but I can't promise anything. I'm not a programmer and I didn't write the software that runs the image gallery.