Exemplo simples:
.any-tag {
width: 100px;
height: 100px;
display: block;
position: relative;
}
.any-tag:after {
width: 100px;
height: 100px;
top: 0;
left: 0;
position: absolute;
z-index: -1;
content: "";
background: url(url-to-your-image);
opacity: .5;
}
Demonstração do Codepen: http://codepen.io/Simek/pen/JfaKr