I would suggest, not knowing exactly what you want, and assuming the following inscription:
<a href="#"><img src="path/to/image.png" /><span>Some text</span></a>
a {
position: relative;
display: inline-block;
}
a span {
position: absolute;
top: 50%;
left: 50%;
margin-top: -1em;
margin-left: -50%;
width: 100%;
height: 2em;
color: #f90;
background-color: rgba(0,0,0,0.5);
}
JS Fiddle demo .
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