Yup. It hasn't happened with art yet, but I had someone copy/paste my entire character profile once and try to RP it as their character because they liked it. I was, of course, furious and they didn't answer their PMs. Eventually, one of my friends found the chat room that they were in and I confronted them publicly about it.
My favorite part was that they were apologetic and didn't know any better because the group of 10+ people they were with supported them stealing entire characters to RP with. Of course I shamed them the most for saying that ignorance is an excuse and not gently correcting them from the get go. Lead by example and all of that. I helped them set up a new character for themselves.
It's made me paranoid to put fully detailed character bios up anywhere, though. As far as art goes, I absolutely recommend you watermark anything and everything. The only things I don't have watermarked are the ones that can be grabbed from DA already without watermarks.