.header-two{
    background: url('../img/mockup.png') no-repeat center center;
    background-size: cover;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    width: 100%;
    height: 100%; /* For at least Firefox */
    position: relative;
}
@media (max-width: 768px) { /* Adjust this breakpoint as needed */
    .header-two {
        background-size: contain;
        height: 14em;
    }
}