No, the image you described was not illegal under "that" law, and you are not addressing what the ruling and law were about.
The Statute criminalized depictions of children engaging in sexually explicit conduct defined as: actual or simulated sexual intercourse, bestiality, masturbation, sadistic or masochistic abuse, and lascivious exhibition of the genitals or pubic area. [18U.S.C. 2256(2)]
Further, child pornography defined under CPPA as: "any visual depiction including photographs, film, video, picture, or computer or computer generated image or picture, whether made by or produced by electronic, mechanical, or other means of sexually explicit conduct where visual depiction is, or appears to be a minor engaging in sexually explicit conduct. [18U.S.C. 2252A(a)(5)(B)]
Ann Coulter is right on the money.
This is where the law failed to pass Constitutional muster. As was pointed out in the majority opinion, there are many mainstream (non-pornographic) works that depict minor characters engaged in sexual activity. When this is done for a movie, as an example, an adult actor is chosen to play the part of the minor character. Under 18USC2252(a)(5)(B), that was illegal, even though no minors were involved. In pointing out that the "children" protected by the statute are cartoons, Rush is right (again).