html,body,div,img {
    margin: 0;
    padding: 0;
}
img{
    width: 100%;
    display: block;
}
.box{
    width: 100%;
    text-align: center;
}
.box img{
    width: 100%;
    height: auto;
    display: inline-block;
    vertical-align: middle;
}
@media screen and (min-width:768px){
    .box img{
        width: 50%;
        height: auto;
    }
}