<a href="http://twitter.com/me" class="twitterbird" title="Twitter link"></a>
use a class for the link itself and forget the div
.twitterbird {
margin-bottom: 10px;
width: 160px;
height:160px;
display:block;
background:transparent url('twitterbird.png') center top no-repeat;
}
.twitterbird:hover {
background-image: url('twitterbird_hover.png');
}