Here's a good technique to display an overlay image that is centered with a semi-transparent background over an image link:
<div class="image-container">
<a class="link" href="#" >
<img class="image" src="/img/thumbnail.png"/>
<span class="overlay-image"><img src="/img/overlay.png"></span>
</a>
</div>
div.image-container{
position: relative;
}
a.link{
text-decoration: none;
position: relative;
display: block;
}
a.link span.overlay-image{
visibility: hidden;
position: absolute;
left: 0px;
top: 0px;
bottom: 0px;
right: 0px;
background-color: rgba(0,0,0,0.2); /* black background with 20% alpha */
}
a.link span.overlay-image:before { /* create a full-height inline block pseudo=element */
content: ' ';
display: inline-block;
vertical-align: middle; /* vertical alignment of the inline element */
height: 100%;
}
a.link:hover span.overlay-image img{
display: inline-block;
vertical-align: middle;
}
a.link:hover span.overlay-image{
visibility: visible;
}