.my-image {
    display: block;
    margin-left: auto;
    margin-right: auto;
    width: 100%;
    height: auto;
    max-width: 220px;   /* dimensione perfetta per col-md-3 */
}

@media (max-width: 768px) {
    .my-image {
        max-width: 180px;  /* mobile */
    }
}