bobomonkey: Actually, the drawing could be libel (although isn't for reasons stated below). Slander is an oral/aural offence. Libel is any material that is published and transmitted between the author and another party or parties.
Both offences have the litmus test of if it's true, and if it damages the reputation or community standing of the agreived party in the eyes of "right minded" (whatever that means) people.
The drawn penis-in-mouth thing clearly doesn't meet either as it's obviously fake, and designed more for satire than for reputational damage.
As Eli said though, 99% of libel cases are frivilous exercises aimed more at wasting your time, energy and resources than actually getting a ruling against you.
Politicians get "Parliamentary / House privilege". It basically means they can say whatever the fuck they like, so long as it's during a session of Government, and the relevant laws aren't applicable. Then, media outlets can report on it, and they're just reporting what happened in a government session.
Unfortunately, privilege has slipped a lot lately and seems to cover a lot outside government sessions now as well.
Example:
There was a really big issue about it in Australia a couple years back when B Heffernan (LP, and flaming nut job) made the most obviously fake receipts for a prostitution service (it's legal here) claiming they were from the office of Justice Kirby (who is an openly homosexual High Court judge). Because they were produced in a session of Parliament, even though it took less than an hour to conclusively prove they were fake, Heffernan didn't face any repercussions because it was all "privileged" away.