Is it illegal to have your favorite "famous animated characters" reproduced on your walls?

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The short answer is: "It depends."

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If you have a legal agreement with the copyright owner of the art that allows you to use their property, then, no, it's not illegal to have your favorite famous mouse, princess, car, robot, etc. painted on your walls, windows and/or whatever. 

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If you do not have a legal agreement, then, yes, it's illegal and unethical. And there can be serious consequences for the copyright infringement.