#sign_in
{
  display: block;
  width: 145px;
  height: 50px;
  background: url("sign_in.png") no-repeat 0 0;

}

#sign_in:hover
{
  background: url("sign_up.png") no-repeat 0 0;
}

#sign_in span
{
  position: absolute;
  top: -999em;
}