.bg{
    background:url(https://source.unsplash.com/random)no-repeat;
    width: 100%;
    height: 100vh;
    background-size: 200%;

}
.form-container{
    position: absolute;
    top: 15vh;
    background: #fff;
    padding: 30px;
    border-radius: 10px;
    box-shadow: 0px 0px 10px 0px #000; 
}
@media only screen and(max-width:678px)
{
    .bg{
        background-size: 300%;
    }
}