th.tablesorter-header {
    background-repeat: no-repeat;
    background-position: center right;
}

th.tablesorter-headerUnSorted {
    background-image: url("img/bg.gif");
}

th.tablesorter-headerAsc {
    background-image: url("img/asc.gif");
}

th.tablesorter-headerDesc {
    background-image: url("img/desc.gif");
}

th.sorter-false {
    background-image: none;
}