#botao_sair
{
  clear: both;
  position:relative;
  display:block;
  height: 30px;
  width: 90px;
  background:url(../images/botao_sair.png) no-repeat;
  background-position:0 0;
  cursor: pointer;
}  
#botao_sair span.hover 
{
  position: absolute;
  display: block;
  height: 30px;
  width: 90px;
  background: url(../images/botao_sair_hover.png) no-repeat;
  background-position: bottom;
}