.image {

  position: relative;

  overflow: hidden;

  width: 200px;

  height: 200px;

  background: url(http://unsplash.it/200/200?image=51);

}

.image::after {

  content: '';

  position: absolute;

  top: -20%; left: -20%; bottom: -20%; right: -20%;

  background: -moz-element(#watermark) center no-repeat;

  transform: rotate(-45deg);

}

#watermark {

  font: 3em sans-serif;

  color: rgba(0,0,0,.5);

  display: flex;

  width: 200px;

  height: 100px;

  justify-content: center;

  align-items: center;

}

#watermark::before {

  content: '©';

}

.hidden {

  height: 0;

  overflow: hidden;

}