If:
X is image width,
Y is image height,
then:
img {
position: absolute;
top: 50%;
left: 50%;
margin-left: -(X/2)px;
margin-top: -(Y/2)px;
}
But keep in mind this solution is valid only if the only element on your site will be this image. I suppose that's the case here.
Using this method gives you the benefit of fluidity. It won't matter how big (or small) someone's screen is. The image will always stay in the middle.