html, body{
    width: 100%;
    height: 100%;
}
body{
    display: flex;
    align-items: center;
}
body, img {
    max-width: 100%;
    max-height: 100%;
}

@media (max-width: 801px) and (orientation: portrait){
    .img2 {
        max-width: 70%;
        height: auto;
        margin: auto;
    }

    .img1 {
        display: none;
    }
}

@media (min-width: 800px) and (orientation: portrait) {
    .img1 {
        max-width: 100%;
        height: auto;
    }

    .img2 {
        display: none;
    }
}

@media (min-width: 401px) and (orientation: landscape) {
    .img1 {
        max-width: 100%;
        height: auto;
        margin: auto;
    }

    .img2 {
        display: none;
    }
}