image.css

div {
    position: relative;
}
span {
    background-color: black;
    background-position: center;
    background-repeat: no-repeat;
    background-size: 50% 50%;
    cursor: pointer;
    height: 32px;
    opacity: 0%;
    position: absolute;
    top: calc(50% - 16px);
    width: 32px;
}
span.not-allowed {
    cursor: not-allowed;
}
ul:hover ~ span {
    opacity: 20%;
}
span:hover {
    opacity: 100%;
}
span:first-of-type {
    background-image: url("img/left.svg");
    left: 0;
}
span:last-of-type {
    background-image: url("img/right.svg");
    right: 0;
}

ul {
    margin: 0;
    padding-left: 0;
}
li {
    display: none;
}
li.active {
    display: block;
}
img {
    display: block;
    width: 100%;
}