Symbolism, Imagery, Allegory
Andy's girlie posters represent a sort of hope, the same way the beer and the library do, but they also symbolize freedom (Which to Andy is almost the same thing.).
You look at those pretty women and you feel like you could almost not quite but almost step right through and be beside them. Be free. (221)
There are worse ways of symbolizing hope than a pretty girl, even if she is just a poster. Hey, we're not here to judge—but there is more to those pin-ups than just hope. There's secrecy. They hide Andy's giant escape hole, after all, and unlike real people, they're very good at keeping their mouths shut. They're so good at it, in fact, that no one finds out what they're hiding until Andy has already put on his boogie shoes and split.