Not sure if it would work for all examples but when I have used watermarks I find covering the whole image in a repeat of my name for example but overlayed "screen" to a level where it is visible but not that destructive can work quite well :)
If someone is shameless enough to use stolen art for an application I don't think they would be stopped by watermarks. In worst case they just copy your whole Portfolio including your watermark and still claim it as their own. In case of PyrZern example they could just change the name and keep the big A3D as it is.