HOpe this helps you.
HTML:
<img src="http://lorempixel.com/400/200" />
<div class="box-transparent">
This is some demo Text
</div>
CSS:
.box-transparent {
position: fixed;
top: 20px;
left: 20px;
background-color: rgba(100, 100, 100, 0.6);
padding: 20px;
}
The important thing is the rgba(100, 100, 100, 0.6); line. 0.6 is the box's opacity.