Watermark your sig a bit. It won't hurt and we'll all know it's because people want to steal it.
Although, I don't have a clue to what your photos are. I saw that chick plenty of times at Deviant art, or we're not talking about any particular picture in your signatures?
Personally, I do think eryoune has thier own style, and I do think there are many good signature makers out there that have pretty amazing skill - however no one is a god, and personally I don't patent any of my stuff, cause atleast 50% of the sig is not my own work =D As far as watermarking it, I would only do it if it is genuinely changed enough to be your own work...
If you post something on the internet, you can expect at least one person to save it to their computer. Only way to avoid it, is, well... Don't post it on the internet. It's like leaving a pie out on the windowsil.
You create art for other people to enjoy, that's the point.