The "fan art" you created are actually "derivative works" from copyrighted works (this includes montages, etc.). Derivative works are just as protected as the original work. Although there are "fair use" exceptions that allow you to use copyrighted works (e.g. for educational purposes, parody, etc.), making $$$ from someone else’s copyrights (as in ad revenue on a web site) without their written permission is always illegal.
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