immagini roll over

<div class="button_uno"><a href="#"></a></div>

.button_uno {
width: 200px;
height: 30px;
}
.button_uno a {
display: block;
width: 200px;
height: 30px;
background: url("../files/blu.gif") no-repeat top left;
}
.button_uno a:hover {
background-position: top right;
}