I went to the link markf
posted. Based on that, my own conclusion would be that they didn't report it. Now I could be wrong (and hope I'm not, obviously) but I read the page that he linked to from top to bottom 3 times. And everything in it talked about images of a minor or of someone who appears to be a minor.
In the eyes of the law, Harry Potter is not a minor. Harry Potter is a character in a book. My understanding of that page is that images of fictional characters, even if they otherwise meet all of the relevant criteria (which the pics in question didn't) do not fall under the federal definition of child pornography.
That being said, it is, I suppose, possible that some state laws might be different, but I don't know what LJ's obligation would be to report something to the state law enforcement agencies if it isn't a violation of the federal law. Being that they didn't mention the states when they mentioned the federal, I am assuming that there isn't one.