table.tablesorter thead tr .header {
    background-image: url(bg.gif);
    background-repeat: no-repeat;
    background-position: -4px;
    cursor: pointer;
}
table.tablesorter thead tr .headerSortUp {
    background-image: url(asc.gif);
}
table.tablesorter thead tr .headerSortDown {
    background-image: url(desc.gif);
}
table.tablesorter thead tr .headerSortDown, table.tablesorter thead tr .headerSortUp {
background-color: #8dbdd8;
}