Or, similar to erikkallen's idea, make the background of the A tag black, and make the image semitransparent on mouseover. That way you won't have to create additional divs.
Source for the CSS-based solution:
a.darken {
display: inline-block;
background: black;
padding: 0;
}
a.darken img {
display: block;
-webkit-transition: all 0.5s linear;
-moz-transition: all 0.5s linear;
-ms-transition: all 0.5s linear;
-o-transition: all 0.5s linear;
transition: all 0.5s linear;
}
a.darken:hover img {
opacity: 0.7;
}
And the image:
<a href="http://google.com" class="darken">
<img src="http://www.prelovac.com/vladimir/wp-content/uploads/2008/03/example.jpg" width="200">
</a>