Let me get this straight.
Don't be a bitch and copy someone's art without permission and credit. By "art" I mean anything such as drawings, writing, photos, edits, etc etc etc. It's not fair. I've had it happen to me a couple of times. Let me tell you, I was very pissed. Very.
But, at the same time, don't post your shit without putting a watermark or signature on it. Someone will steal it at some point and claim it as theirs. Don't think so? Fine. I know for a fact that there are some websites out there that steal an artists' work, illegally, without their permission or their knowing. People are currently trying to shut them down.
Is it that hard to download a free photo editing app and type in your watermark? Or your signature, username, whatever? It should not be. How would you feel if someone stole your stuff at school? And no one paid attention to you? Terrible, yeah? That's how we feel. Terrible. Like our work doesn't mean a damn thing. Even if we put our signatures on our work.
The internet is a great place but, at the same time, there are some shitwhisks out there who don't give a crap about people or their feelings. (Excuse the whisk reference). So, yeah. Be careful. And don't steal someone's stuff. Okay.