bootstrap pictures overlapping
<p>I'm fairly new to Html &amp; Css. I'm trying to make a portfolio site as an exercise for myself. However, when I try to put multiple pictures next to eachother, sort of like a gallery. My pictures start to overlap eachother. I have tried to find a solution for a few hours now. But nothing seems to work.</p>

<p>Does any of you guys have a suggestion that might lead to a solution?
Thanks in advance!!]<a href="http://i.stack.imgur.com/JmIwb.jpg" rel="nofollow">1</a>   </p>

<pre><code>&lt;!DOCTYPE html&gt;
&lt;html lang="en"&gt;
&lt;head&gt;
    &lt;meta name="viewport" content="width=device-width", initial-scale=1.0&gt;

     &lt;!-- Latest compiled and minified CSS --&gt;
&lt;link rel="stylesheet" href="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css"&gt;


    &lt;link href="styles/styles.css" rel="stylesheet"&gt; 
    &lt;link href='https://fonts.googleapis.com/css?family=Roboto:400,700' rel='stylesheet' type='text/css'&gt;


    &lt;title&gt;Noah Wallaart&lt;/title&gt;
&lt;/head&gt;
&lt;body&gt;

&lt;!-- jQuery library --&gt;
&lt;script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"&gt;&lt;/script&gt;

&lt;!-- Latest compiled JavaScript --&gt;
&lt;script src="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/js/bootstrap.min.js"&gt;&lt;/script&gt;


&lt;div class="header"&gt;  
        &lt;div class="navbar navbar-default"&gt; 
           &lt;div class="container"&gt;

            &lt;/div&gt;  
        &lt;/div&gt;  
 &lt;/div&gt;

 &lt;div class="jumbotron"&gt;
     &lt;div class="container"&gt;
         &lt;div class="row"&gt;
                &lt;div class="col-md-6"&gt;     
                      &lt;h1&gt;Noah Wallaart&lt;/h1&gt;
                &lt;/div&gt;

                &lt;div class="col-md-6"&gt;    
                &lt;/div&gt;      
         &lt;/div&gt;  
     &lt;/div&gt;
&lt;/div&gt;     

&lt;section&gt;
    &lt;div class="container"&gt;
       &lt;div class="row"&gt;
           &lt;div class="col-md-"&gt;&lt;img src="image/background.JPG"&gt;&lt;/div&gt;
           &lt;div class="col-md-3"&gt;&lt;img src="image/background.JPG"&gt;&lt;/div&gt;
           &lt;div class="col-md-3"&gt;&lt;img src="image/background.JPG"&gt;&lt;/div&gt;
           &lt;div class="col-md-3"&gt;&lt;img src="image/background.JPG"&gt;&lt;/div&gt;
       &lt;/div&gt;

    &lt;/div&gt;

&lt;/section&gt;



&lt;/body&gt;
&lt;/html&gt;
</code></pre>