This is why I rarely post images of my art on any social media site. It's way too easy for someone to steal it. I remember a few years back a professional artist posted some wip images to Facebook. Someone stole the concept and sold it to a well known collectibles co. who proceeded to make figurines based on this artist's paintings. She had to get a lawyer and take them to court to get them to stop making them.

I've had people on tumblr steal pictures I took at a Black Veil Brides concert.
Not only steal them, but watermark them themselves and try to pass it off as their own.
It pissed me off to no end and since then I've never posted any other photography work of mine to tumblr, because those people are rude and can not be trusted with anything.
Half of me says it's because I was stupid and didn't watermark it myself, but the other part of me says people shouldn't be assholes and claim other peoples work as their own.
The world is full of garbage.

I have had my art stolen only once that I know of. It was many years ago when I used devianart quite often. The person had taken my drawing of an oc. It was an Alice in wonderland themed drawing, and pretty generic to be honest. They had just reposted it without even removing my signature. So I was contacted by someone who saw it was stolen. It was taken down very quickly after I reported the picture.

I know on DA if the original creator reports the drawing as stolen it gets taken down quicker than if someone else does it. At least it did back when I used it.

once i had to chase someone down for using one of my friend's oc art for their own oc... it was ridiculous.