Dejo el resto del codigo.. quizá desde ahi sea mas fáçil de centrar
Código CSS:
Ver original#menu1 ul { margin:0; list-style:none;}
#menu1 li {display:inline;margin:0;padding:0; }
#menu1 a {
text-shadow: 0px 2px 3px #ffffff;
display:block;
float:left;background:url(http://4.bp.blogspot.com/-M3F4NPGILtw/UHRdcstXd3I/AAAAAAAAAXo/vsvytpUEGno/s1600/comprar_portatil_barato_main.png) no-repeat left top;margin:0 1.5px 0 1.5px;padding:0 0 0 6px;text-decoration:none;}
#menu1 a span {float:left;display:block; background: url(http://3.bp.blogspot.com/-wKy1HpN2soU/UHRYMnPSivI/AAAAAAAAAXY/JwzWt0B7YgQ/s320/comprar_portatiles_muy_baratos_main.png) no-repeat right top;padding:5px 16px 4px 12px;color:#999;}
#menu1 a span {font: bold 14px Arial, Helvetica, sans-serif; color:#000099; float:none;}
#menu1 a:hover span {color:#000099;}
#menu1 a:hover {background-position:0% -27px;}
#menu1 a:hover span {background-position:100% -27px;}
.clear {clear:left}