if your image is purely decorative, then it might be a more semantic solution to use it as a background-image. You can then specify the position of the background
background-position: center center;
If it is not decorative and constitutes valuable information then the img tag is justified. What you need to do in such case is style the containing div with the following properties:
div{
display: table-cell; vertical-align: middle
}
Read more about this technique here. Reported to not work on IE6/7 (works on IE8).