How do I set transparency for background-image?

Title, now is not able to change the transparency of the image itself, the picture is written in Background-image CSS, do not know if there is a way to set his transparency?

div{    position:relative;        } div:after{    content: ";    Display:block;    width:100%;    height:100%;    Position:absolute;    top:0;    right:0;    bottom:0;    left:0;    Background:url (...);    opacity:0.5;}

Background map with PNG, with transparency.

The element sets transparency when the background, another element obscures to the top of the guest under its son, pretends to be the content of this element.

CSS's Opacity property allows you to set the transparency properties of the image.

#div {    background-image:url (img.png);    opacity:0.4;    Filter:alpha (opacity=40);/* for IE8 and earlier versions */}

Cannot change the transparency of the image itself, it can only change the transparency of the entire div to achieve a similar visual effect:
1.opacity Properties
2. Use the CSS3 filter-webkit-filter (as opposed to the previous IE filter) if compatibility allows.

Look at the implementation of this site

Opacity in CSS background images