.bootbox.modal .modal-content::after {

    content: "";
    display: block;
    position: absolute;
    background: url("/static/images/paint-bamboo-mountain.png");
    width: 100%;
    height: 100%;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    opacity: 0.2;
    background-repeat: no-repeat;
    background-size: cover;
    z-index: 0;
}
.modal-body, .modal-footer {
    z-index: 1;
}