height: 100 
 height: 80 
 height: 120 

Center an image inside a div, vertical and horizontal, without knowing the image's size

<div>&nbsp;<img src="placeholder.gif" width="70" height="120" />&nbsp;</div>
<div>&nbsp;<img src="placeholder.gif" width="90" height="80"  />&nbsp;</div>
<div>&nbsp;<img src="placeholder.gif" width="70" height="120" />&nbsp;</div>

div {
	float: left;
	text-align: center;
	width: 150px;
	height: 150px;
	margin: 5px;
	border: 1px solid #ccc;
	font-size: 1em;
	line-height: 148px;
	}

div img {
	margin-top: expression(( 150 - this.height ) / 2); 
	}

html>body div img {    /*hidden from IE 5-6 */
	margin-top: 0; /* to clean up, just in case IE later supports valign! */
	vertical-align: middle;
	}
	
Note: some <script> tag, either inline or external (can be a dummy one), is needed to get IE 5.0 on track.

* Extended from Steve Clay's http://mrclay.org/web_design/centered_image/

* Note: CSS expression should be hidden in IE contidional comments to get clean valid css file.

* Note 2: &nbsp; is requiered before and after the image for proper display in IE Mac and Opera.