Artist, PLEASE, for all that is holy to you, put a watermark or AT LEAST a tag on ANY and ALL art you post online!
I don't care if you think it's something silly. If it's something you personally wouldn't or couldn't sell. I don't care if it's a WIP, a tattoo design, a comic, WHATEVER! It's going to get shared around. It's going to be dowloaded, screenshotted, uploaded, reposted, texted, tweeted, and sent out into the world and seen by hundreds, maybe millions of people.
Get that free advertising on your stuff. Get those sweet new followers, customers, and supporters. DON'T make it easy for someone to steal your art, to sell it on a phone case, to list it as a shirt for sale, to use in their video, WITHOUT your mark on it!
Please! Oh the number of amazing pieces of art I've seen blatantly stolen. The images reposted and shared but uncredited. The awesome artwork downloaded and saved for later but without any name or credit on them!