.assin a{
 position:relative;
 top:20px;
 display:block;
 height:auto;
 background-image:url(../img/assinatura.png);
 background-repeat: no-repeat;
 margin-right:0px;
}
.assin a:hover{
 background-position: bottom;
 background-repeat: no-repeat;
 *top:20px;
}