html
<div class="span4 red">
<img src="http://img.thesun.co.uk/aidemitlum/archive/01500/SNE0125Q---_149991_1500286a.jpg">
<div class="overlay">
<span>Text text text</span>
<button>Button</button>
</div>
</div>
css
.span4 {
width:25%;
display:block;
position:relative; /*added*/
}
/*added*/
.overlay {
position:absolute;
top:0;
width:0;
width:100%;
height:100%;
background-color:rgba(0, 0, 0, 0.3);
display:none;
}
.red:hover .overlay {
display:block;
}