With the Opacity tag image will be transparent in css.
Here is the example:
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Untitled Document</title>
<style>
#img {
opacity:0.4;
}
</style>
</head>
<body>
<div id="img">
<p><img src="butterfly.jpg" width="300" height="231" alt="butterfly in saanich" /></p>
<p><img src="butterfly.jpg" width="300" height="231" alt="butterfly in saanich" /></p>
<p><img src="butterfly.jpg" width="300" height="231" alt="butterfly in saanich" /></p>
<p><img src="butterfly.jpg" width="300" height="231" alt="butterfly in saanich" /></p>
</div>
</body>
</html>
