HTML求助！代码和示例图如下，请问如何使得图片标注的五张图片垂直居中？

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

无标题文档 header{background:#09F; height:50px; width:100%; line-height:20px;} aside{background:#C6F; height:850px; width:18%; float:left;} ul{list-style-type:none;} article{height:800px; width:65%; float:left;} footer{background:#99F; width:100%; height:50px; clear:left;}