/* Remove this if you dont want background overlay over the image or adjust per your needs */
body.login_admin:before {
    content: " ";
    background: rgba(31, 31, 31, 0.7);
    min-height: 100%;
    min-width: 100%;
    position: absolute;
}
body.login_admin {
    background: url('https://crm.waw-it.com/media/login_customers.jpg') no-repeat center center fixed; /* The full image url*/
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
}