Put your image in a container div then use the following CSS (changing the dimensions to suit your image.
.imageContainer{
position: absolute;
width: 100px; /*the image width*/
height: 100px; /*the image height*/
left: 50%;
top: 50%;
margin-left: -50px; /*half the image width*/
margin-top: -50px; /*half the image height*/
}