I came across this today: https://davidwalsh.name/image-security

Basically, a 3rd party service (http://cloudinary.com) will watermark your images however you like, on-the-fly. There are loads of options and my favourite is the "invisible" watermark using low opacity in the blue channel which is hard for the human eye to see, and survives a screengrab as well.

The free plan allows for 75,000 images, 2Gb storage and 5Gb monthly bandwidth limit (more than enough for most here). Worth a look if you're paranoid about folks stealing your images...