1. This is yet another cartoon about the MPs' expenses scandal. One of the most outrageous expenses claims was made by Tory MP Sir Peter Viggers, who submitted an invoice for a duck house to the Commons fees office, claiming £1,645 as part of his MP's expenses. This story has captured the imagination of the press and cartoonists alike (for example, The Independent's Dave Brown and Mac in the Daily Mail).
2. If insults or criticism are '(like) water off a duck's back' to someone, it has no effect on them. • He's always being told he's lazy and incompetent, but it's just water off a duck's back to him. EnglishClub.com has an article about this expression including this explanation of its origin:
Probably related to the fact that ducks have oily feathers and water can't get through them, so water runs off their backs. In the same way, criticism can either get through to someone and upset them, or not get through to them and not upset them, and be "like water off a duck's back."
3. The cartoonist has put the adjective 'lame' before duck. 'Lame' is an adjective used to describe a person or animal unable to walk properly because one of their legs has been injured. However, the expression 'lame duck' describes 'a politician whose power has been diminished because he or she is about to leave office'; e.g. Gordon Brown is a lame duck prime minister who has no chance of winning the next election.