灰化图片，greyscale image

<!-- HTML -->
<a class="a-thumb"><img src="http://www.cnblogs.com/img/proxy-thumb.gif" /><span> </span></a>
 
/* CSS */
a.a-thumb {
        border: 0px solid black;
        position: relative;}
a.a-thumb img {
        width: 60px;
        
        border: 0;}
a.a-thumb span {
        background-color: #000000;
        position: absolute;
        top: 0;
        left: 0;
        width: 60px;
        
        z-index: 100;
        filter: alpha(opacity=20);
        -moz-opacity: 0.2;
        opacity: 0.2;}