爲什麼如果一個div包含一個div沒有高度的圖像？

我嘗試做如下頁面：爲什麼如果一個div包含一個div沒有高度的圖像？

<html> 
    <head> 
     <meta charset = "utf-8"> 
     <link href="estiles.css" type="text/css" rel="stylesheet"> 
    </head> 
    <body> 
     <div> 
      <h1> GALERY OF IMAGES</h1> 
     </div> 
     <div id = "leftDiv"> 
      hola 
     </div> 

     <div id = "galery"> 
      <div id = "centerGalery"> 
       <div><img class = "imageGalery" src = "http://marcus.ridoutfamily.co.uk/wp-content/uploads/2013/08/78186357b7074a1a3bda5d3c0ed4a2e0.jpg"></div> 
       <div><img class = "imageGalery" src = "http://marcus.ridoutfamily.co.uk/wp-content/uploads/2013/08/78186357b7074a1a3bda5d3c0ed4a2e0.jpg"></div> 
       <div><img class = "imageGalery" src = "http://marcus.ridoutfamily.co.uk/wp-content/uploads/2013/08/78186357b7074a1a3bda5d3c0ed4a2e0.jpg"></div> 
       <div><img class = "imageGalery" src = "http://marcus.ridoutfamily.co.uk/wp-content/uploads/2013/08/78186357b7074a1a3bda5d3c0ed4a2e0.jpg"></div> 
       <div><img class = "imageGalery" src = "http://marcus.ridoutfamily.co.uk/wp-content/uploads/2013/08/78186357b7074a1a3bda5d3c0ed4a2e0.jpg"></div> 
       <div><img class = "imageGalery" src = "http://marcus.ridoutfamily.co.uk/wp-content/uploads/2013/08/78186357b7074a1a3bda5d3c0ed4a2e0.jpg"></div> 
       <div><img class = "imageGalery" src = "http://marcus.ridoutfamily.co.uk/wp-content/uploads/2013/08/78186357b7074a1a3bda5d3c0ed4a2e0.jpg"></div> 
       <div><img class = "imageGalery" src = "http://marcus.ridoutfamily.co.uk/wp-content/uploads/2013/08/78186357b7074a1a3bda5d3c0ed4a2e0.jpg"></div> 
      </div> 
     </div> 
    </body> 
</html>

中，我有一個頭和兩個divs，一個在左，一個在右。每個div都有身體寬度的50％。這裏有任何問題。

問題來了，當我想添加一些圖像作爲在它的身體的權利的div中的照片集合。我希望所有的圖像將集中在galery div內，但我無法實現（我創建了一個新的div來保存所有圖像）。相反，我看到我存儲每個圖像的div沒有高度。

這是正常的嗎？如果它包含在div中的圖像，div沒有高度？

這裏是我的CSS：

body{ 
    text-align: center; 
    margin: 0px; 
    padding: 0px; 
} 

#leftDiv{ 
    float: left; 
    width: 50%; 
    background-color: green; 
} 

#galery{ 
    float: right; 
    width: 50%; 
    background-color: blue; 
} 

#centerGalery{ 
    width: 100%; 
    margin: 0 auto; 
} 

.imageGalery{ 
    height: 100px; 
    width: 100px; 
    float: left; 
}

這就是我的意思是：提前

謝謝！