how to adjust background image opacity in css code example

Example 1: set background image opacity

#bg{
background-image: url('images/wood1.jpg');
opacity:0.2;
width:300px;
height:300px;
}

Example 2: css opacity example

.opacity30 {
  opacity: 0.3;
  filter: alpha(opacity=30); /* For IE8 and earlier */
}