<style>
}          
.gallery ul li {          
width:200px; /* a half of image width */          
height:200px; /* a half of image height */          
margin:0 10px 10px 0;          
float:left;          
position:relative;          
}          
.holder {          
position:absolute;          
top:0;          
left:0;          
margin:-100px 0 0 -100px; /* margin-left: a half of 'li' width and margin-top: a half of 'li' height */          
}          
</style>