Is animated child illegal?

I have heard that you can be arrested for possession of child if you download an animated picture depicting a person under 18 in a ually expliced manner. This frightens me because sometimes a buddy will email me a comic picture of Bart Simson or Charlie Brown, getting down to buisness with some cartoon girl, and I never gave it a second thought. It's adult humor, in my oppinion, and worth a few laughs. I'd hate to think that I was violating a law by having something like that. I know I'm not a pediphile. I would hate to think that I could be treating like one over comics. Also, I've seen Hentai where the charaters seem to be under 18, and again I never gave it a secong thought because these are cartoons, but from what I've been finding out is that "sim" child is as bad as the real thing and just as illegal. I'm guessing that with CG animation advancing the way it is that soon people will hardly be able to tell the difference. But what are that facts?