html,body{
    padding: 0;
    margin: 0;
 height: 100%;
    width: 100%;
}
@media (min-width: 768px)
{
    body{
        background: url("web.jpg") no-repeat;
        background-size: cover;
        background-position: bottom;
    }
}
@media (max-width: 767px)
{
    body{
        background: url("mobil.jpg") no-repeat;
        background-size: cover;
        background-position: bottom;
    }
}