body {
    background:#ccc;
}
h1 {
    color: red;
}

img.small-photo {
    width: 300px;
}

#hidden_img {
    display: flex;
    justify-content: center;
    position: absolute;
    width: 100%;
    z-index: 100;
    background-color: aqua;
}

.big-img {
    width: 900px;
    margin: 0 auto;
}