.gallery_back_button span.back_btn {
    background: url('images/icons/arrow_prev.jpg') top left no-repeat;
    display: block;
    height: 30px;
    width: 30px;
}
.gallery_back_button span.back_btn:hover {
    background: url('images/icons/arrow_prev_hover.jpg') top left no-repeat;
}