The simpler, the best:
img {
top: 0;
bottom: 0;
left: 0;
right: 0;
margin: auto auto;
position: absolute;
}
Then you need to insert your img tag into a tag that sports position:relative property, as follows:
<div style="width:256px; height: 256px; position:relative;">
<img src="photo.jpg"/>
</div>