"There is no CSS property background-opacity, but you can fake it by inserting a pseudo element with regular opacity the exact size of the element behind it."

div {
width: 200px;
height: 200px;
display: block;
position: relative;
}

div::after {
content: "";
background: url(image.jpg);
opacity: 0.5;
top: 0;
left: 0;
bottom: 0;
right: 0;
position: absolute;
z-index: -1;
}