*{
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}
body{
    display: flex;
    justify-content: center;
    align-items: center;
    flex-direction: column;
    height: 100vh;
    position: relative;
    background: linear-gradient(to bottom right, #2389da, #74ccf4, #5abcd8);
}
#myImg{
    position: relative;
    width: 800px;
    height: 450px;
    box-shadow: 10px 10px 5px rgba(0,0,0, .4);
    transition: .3s;
}
body::before{
    content: '';
    width: calc(800px - 30px);
    height: calc(450px - 30px);
    border: 3px solid #e0ffff;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    z-index: 9999;
}
@media(max-width:767px){
    #myImg{
        width: 300px;
    }
    body::before{
        width: calc(300px - 30px);
    }
}