The only real way for anyone to really know its yours is either to have your signature/name on it. Thats how real artist do it too.
Stock is stock. The original owners of said stock are the original artist. Hardly anyone on the internet actually made the original, so feel free to rip it.
Renders are the same.
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Really though signatures that just involve stock manip or some shit are pretty lame. Its usually if not always just a tad bit better, or a tad bit worse than the original.
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Ripping is easy for me.
1. Does it have your name on it?
2. Do you take credit for making it?
Thats it. I don't give a shit if you didn't make it, but don't take credit for it if you didn't.