A great way to find out who stole your art, is to go on to Google Image Search, and simply drag-and-drop your search image onto the search area.
Google returns all instances of images that "looks like" what you asked it to find.

I've known about it, but I hadn't tried it until now.
The results were ... pretty interesting. It turns out a lot of my stuff HAS been stolen and posted elsewhere.

By "stolen", I mean posted elsewhere without my knowledge or consent, even if they give me credit (which they almost never do).
Whining about that might be a moot point in this day and age of the "information wants to be free yo" internet, but still ... that technically counts as stealing, you know.

I'm willing to call it a badge of honor that someone thought it was worth the effort to copy and post it to their Tumblr (which I personally consider the ultimate rip-off-the-internet machine btw) ... so far.  But that's for me and me alone to decide, and also my decision to make about when to overturn that and start calling out people for stealing my shit.

Would I be an asshole for doing that? Maybe.
Do I have the right to do that? You betcha.

I've been kinda lazy about putting in that ugly watermark on dA posts as of late, but maybe I need to start putting it in again ...