.image-item img {
    width: 60%;
    margin: 20%;
    background: white;
}
.contenedorUnionImagenes {
    position: absolute;
    margin: 10% 13.5%;
    width: 73%;
    height: 50%;
    border: 2px dotted orange;
}
@media (max-width: 992px) {
    .contenedorUnionImagenes {
        margin: 18% 25%;
        width: 50%;
        height: 75%;
    }
}
@media (max-width: 568px) {
    .image-item img {
        width: 90%;
        margin: 20% 5%;
    }
}