gistfile1.txt

HTML:
<div class="watermarked"><img src="bild.jpeg" /></div>

CSS:
.watermarked {
  position: relative;
}
.watermarked:after {
  content: "Copyright";
  display: inline-block;
  width: 100%;
  height: 100%;
  position: absolute;
  background: #aaa;
  opacity: 0.7;
  top: 0px;
  left: 0px;
}