In the shawshank redemption why does Andy ask red for a rock hammer?

In the movie The Shawshank Redemption, Andy Dufresne (played by Tim Robbins) does not ask Red (played by Morgan Freeman) for a rock hammer. Instead, he asks for a poster of Rita Hayworth, a famous Hollywood actress. Andy needed the poster to hide a hole he had dug in his cell wall to escape from Shawshank State Penitentiary.