.gallery-item:nth-child(1) .image {
    background-image: url('../img/slider_1.png');
}
.gallery-item:nth-child(2) .image {
    background-image: url('../img/slider_2.png');
}
.gallery-item:nth-child(3) .image {
    background-image: url('../img/slider_3.png');
}
.gallery-item:nth-child(4) .image {
    background-image: url('../img/slider_4.png');
}