Ed, I'm not a lawyer and certainly not an expert in defamation... but I'm not sure you have "slander" right. My impression is that slander is transitory defamation... basically verbal in nature whereas libel is something more fixed, like published materials. I think the pic of the politicial with the open fly would be news, but if captioned "politician tries to increase votes by opening the barn door", then maybe it would be libel.
By the way, it seems like a lot of defamation threats (if not real actions) are the result of embarassment. To the best of my knowledge, embarassing someone isn't a crime unless they have really been harmed by that embarasment.