body {
    background-color: #f9f9f9;
}

img {
    display: block;
    margin-left: auto;
    margin-right: auto;
    width: 100%;
}

@media screen and (min-width: 600px) {
    img {
        width: 85%;
    } 
}

@media screen and (max-width: 599px) {
    img {
        width: 100%;
    } 
}