[kulbabka]

Сработало так

window.onload = function(){
	var elements = document.getElementById('list-holder').children;
	for(var j = 0; j < elements.length; j++){
		var item = elements[j];
		var img = item.children[0];
		img.style.marginLeft = ((item.offsetWidth - img.width)/2) + 'px';	
		img.style.marginTop = ((item.offsetHeight - img.height)/2) + 'px';
	}
};