.bxslider li.slider-01:before {
    background-image: url(https://nk-techno2017.co.jp/system_panel/uploads/images/260903_3.PNG);
    background-position: center;
    background-size: cover;
}

.bxslider li.slider-02:before {
    background-image: url(https://nk-techno2017.co.jp/system_panel/uploads/images/260903_4.PNG);
    background-position: center;
    background-size: cover;
}