CSSでabsolt を上下センター　中心

http://bashalog.c-brains.jp/14/08/06-100100.php
https://hyper-text.org/archives/2014/08/position_absolute_center_layout.shtml

position: absolute;
top: 0;
left: 0;
right: 0;
bottom: 0;
margin: auto;
width: 任意の値
height: 任意の値

#img_box2 .navi-image a .testa img{position: absolute;
	top: 0;
left: 0;
right: 0;
bottom: 0;
margin: auto;
}