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>