That Andy Dufresne looooves his posters. But what—no war films? No scenes from a courtroom drama? Why's it always gotta be the half-naked ladies?
We get the sense that Andy isn't a total perv, so it's odd that he keeps insisting on the girlie posters. On the surface, it's probably because he's smart, and doing whatever he can not to arouse suspicion. Men in prison get lonely, and it only makes sense they'd want a woman to keep them company. Whereas if the warden noticed he had a poster hanging on his wall of Buckingham Palace or a landscape of coastal Oregon…his antennae might go up.
On a deeper level, the subject of these posters symbolizes what isn't shown just as much as—if not more than—what is shown. The women in those posters are all revealing just enough to be tantalizing, while not exposing very much at all. Same with the tunnel right behind them; it's there, waiting to be found, but still totally hidden. What a sexy tunnel it is. The sexy starlets on the posters are a glimpse of the world beyond, just out of reach but not out of mind.