*{
    margin: 0px;
    padding: 0px;
}
body{
    height: 100vh;
    width: 100vw;
    background-color: #ff1824;
}

img.centred{
    height: 100vh;
    position: absolute;
    margin: auto;
    left: 0; 
    right: 0;
    top: 0;
    bottom: 0;  
}
@media (max-width: 800px) {
    img.centred{
        height: auto;
        width: 100vw;
    }
}