change background opacity js 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 set background opacity

/*background color opacity with explainations */
rgba(51, 170, 51, .1)    /*  10% opaque green */ 
rgba(51, 170, 51, .4)    /*  40% opaque green */ 
rgba(51, 170, 51, .7)    /*  70% opaque green */ 
rgba(51, 170, 51,  1)    /* full opaque green */