footer#footer::after {
    background-image: url(resimurl);
    background-size: cover;
    background-position: center;
    width: 100%;
    height: 100%;
    content: '';
    position: absolute;
    left: 0;
    top: 0;
    z-index: -1;
    opacity: .4;
}

.footer--section {
    position: relative !important;
    z-index: 1;
}