If you copy a picture of Spongebob My computer teacher said " You will not get in trouble because Sponge Bob is a popular character and people will know your not the creator of the cartoon."
Lets say a cartoon is not a famous or popular character and you make a cartoon out of that character and you say to Disney Studios " I want to make a cartoon out of this characters." If you make money off of a character which is not yours and you make money off of it and the creator is not making money you can get in big trouble you could go to jail but it's very slim chances you can go to jail.
How ever if you trace a character if it's not famous and you do not make money off of that character you will not get sued. That is what my teachers has said.
For example if you trace a Skeleton on the internet you can not get sued because every body has a skeleton inside of their body and I trace stuff but I do not make money off of it.
Mostly do not copy any stuff and Mostly I draw my own characters. Like I make up my own characters to and I do not just trace other art work.
Public Domain is okay to use because they want you to use their art work. My teachers and Class Mate say it's okay to use fan art But it's like making your own version of other people characters if you do not make any money off of fan art it's probably okay to use but lets say you make a cartoon out of someone eles's character weather it's fan art or not it's not okay to copy other peoples art work.