图片居中不了

<div class="container" id="cont1">
        <div class="row">
            <div class="col-md-4 col-xs-12">
                <img src="images/chrome.png" alt="chrome">
              ...
            </div>
            <div class="col-md-4 col-xs-12">
                <img src="images/firefox.png" alt="chrome">
                ...
            </div>
            <div class="col-md-4 col-xs-12">
                <img src="images/ie.png" alt="chrome">
             ...
            </div>
        </div>