css background image opacity

1background-image: url(IMAGE_URL); /* fallback for older browsers */
2
3background-image: linear-gradient(to bottom, rgba(0,0,0,0.6) 0%,rgba(0,0,0,0.6) 100%), url(IMAGE_URL);
4

1#bg{
2background-image: url('images/wood1.jpg');
3opacity:0.2;
4width:300px;
5height:300px;
6}
7

1body::before{
2    content: "";
3    position: absolute;
4    top: 0;
5    right: 0;
6    bottom: 0;
7    left: 0;
8    z-index: -1;
9    background-image: url(image.jpg); /*Put your image url*/
10    background-size: cover;
11    background-position: center;
12    opacity: 0.25; /*Value from 0.0 to 1.0*/
13}

1/* Two ways to make images opaque */
2
3div {
4	background-color : rgba(120, 120, 120, 0.5) 
5   	/* the 0.5 is the value of opacity on a scale of 0-1 */
6}
7
8/* OR */
9
10div {
11	background-color : blue
12    opacity : 50%
13}

1body { 
2  background-color: transparent;
3}
4

1/*
2  You have to fake it, as there is no such property
3*/
4
5div {
6  width: 200px;  /* Image Width */
7  height: 200px; /* Image Height */
8  display: block;
9  position: relative;
10}
11
12div::after {
13  content: "";
14  background: url(image.jpg);
15  opacity: 0.5;
16  top: 0;
17  left: 0;
18  bottom: 0;
19  right: 0;
20  position: absolute;
21  z-index: -1;   
22}