Resizing images using media queries

@media (max-width:767px) {
    .portfolio img {
        max-width: 100%;
        max-height: 100%;
    }
}

@media (min-width:768px) {
    .portfolio img {
        max-width: 240px;
        max-height: 240px;
    }
}

@media (min-width:992px) {
    .portfolio img {
        max-width: 314px;
        max-height: 314px;
    }
}

@media (min-width:1200px) {
    .portfolio img {
        max-width: 380px;
        max-height: 380px;
    }
}