Inset Box-Shadow At Images

Hey guys,
I've a little workaround for you. I had the problem to get an inset box-shadow at images. Because of this I try something in CSS and this works!

Markup

<div class="thumb">
 <a href="#">
 <img src="img.jpg" alt="avatar" title="bla">
 </a>
</div>

CSS

.thumb {
 a {
 position: relative;
 line-height: 0;
 display: inline-block;

 &:before {
 content: '';
 position: absolute;
 top: 0;
 bottom: 0;
 left: 0;
 right: 0;
 -webkit-box-shadow: (inset 0 1px 5px rgba(0,0,0,.55);
 -moz-box-shadow: (inset 0 1px 5px rgba(0,0,0,.55);
 box-shadow: (inset 0 1px 5px rgba(0,0,0,.55);
 }
 &:hover {
 opacity: .7;
 }
 }
}