[css] How to Decrease Image Brightness in CSS

OP wants to decrease brightness, not increase it. Opacity makes the image look brighter, not darker.

You can do this by overlaying a black div over the image and setting the opacity of that div.

<style>
#container {
    position: relative;
}
div.overlay {
    opacity: .9;
    background-color: black;
    position: absolute;
    left: 0; top: 0; height: 256px; width: 256px;
}
</style>

Normal:<br />
<img src="http://i.imgur.com/G8eyr.png">
<br />
Decreased brightness:<br />
<div id="container">
    <div class="overlay"></div>
    <img src="http://i.imgur.com/G8eyr.png">
</div>

DEMO