table thead tr th {
    background-image: url('bg.png')!important;
    background-size: 10px;
    background-repeat: no-repeat;
    background-position: right center;
}
table thead tr th.sorting-desc {
    background-image: url('desc.png')!important;
}
table thead tr th.sorting-asc {
    background-image: url('asc.png')!important;
}