I'm so sorry this painting has been stolen so often. But goes to show how incredible the deft style and lighting is! Toothless' expression is deceptively simple, a little cute but a lot of mean. I love it!
I'm sorry to hear that your beautiful art is being stolen. I suggest that you put your signature/mark some where, where it won't be cut off as easily. Putting it on the bottom corner can be cropped out, but if you put it in a place like right beside his wings or somewhere near him would fix that problem. Not saying people still won't try to steal it, but it's a start . Sad to say I'm even giving this advice to you because I shouldn't have to. People shouldn't be stealing art that they didn't put time and effort into.
Try editing the post so there's a watermark in the image, or even make your own. It will make the image harder for anyone to claim as their own, and much easier to spot a thief if they try to erase it.