OP, your friend brings a good point, a wrong point, but a good point nonetheless. Essentially, the problem lies in that you are owning illegal material that is produced by a very lucrative underground industry. Regardless of wether or not the pictures directly aid the child porn business, the images are still created out of the exploitation of (mostly) non-consenting children. It's the same as knowingly taking an object that you was stolen from another person's home, even if you didn't pay for it. It's still produced out of an illegal action, and thus illegal to own.