<div class="navi"></div>
<div id="infoi">
    <img src="info_icon2.png" height="20" width="32"/>
</div>

利用position relative/absolute在兩個div上後用z-index可以設定元素的z(立體面)

#container {
  width: 100px;
  height: 100px;
  position: relative;
}
#navi,
#infoi {
  width: 100%;
  height: 100%;
  position: absolute;
  top: 0;
  left: 0;
}
#infoi {
  z-index: 10;
}
<div id="container">
  <div id="navi">a</div>
  <div id="infoi">
    <img src="https://appharbor.com/assets/images/stackoverflow-logo.png" height="20" width="32" />b
  </div>