Working in old browsers (IE >= 8)

Automatic margins, paired with values of zero for the left and right or top and bottom offsets, will center an absolutely positioned elements within its parent.

View Result

HTML

<div class="parent">
  <img class="center" src="<http://lorempixel.com/400/200/>" />
</div>

CSS

.parent {
  position: relative;
  height: 500px;
}

.center {
  position: absolute;
  margin: auto;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
}

Elements that don’t have their own implicit width and height like images do, will need those values defined.

Other resources: Absolute Centering in CSS