.nbox img.norm {
    width: 100%;
    max-width: 370px;
    margin: 0 auto;
    cursor: pointer;
}

.nbox img.smallimg {
    width: 100%;
    max-width: 250px;
    display: none !important;
    margin: 0 auto;
    cursor: pointer;
}

@media screen and (max-width: 640px) {
    .nbox {
        text-align: center;
    }
    
        .nbox img.norm {
            display: none !important;
        }

        .nbox img.smallimg {
            display: block !important;
        }
}