CODE EXAMPLE FOR CSS

background image opacity css

body::before{
    content: "";
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    z-index: -1;
    background-image: url(image.jpg); /*Put your image url*/
    background-size: cover;
    background-position: center;
    opacity: 0.25; /*Value from 0.0 to 1.0*/
}